Auto merge of #18145 - tigercosmos:master, r=larsbergstrom
Auto merge of #18145 - tigercosmos:master, r=larsbergstrom

fix windows build issue #18055

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With some reasons, the windows and VS environment do not set the `env("PLATFORM")` variable.
So, there's no need to check the variable, and set `os_type` equl to `pc-windows-msvc` as default. Then the script can get correct url to download `rustc`.

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #18055 (github issue number if applicable).

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KiChjang/servo,pyfisch/servo,emilio/servo,dati91/servo,anthgur/servo,notriddle/servo,SimonSapin/servo,jimberlage/servo,ConnorGBrewster/servo,SimonSapin/servo,emilio/servo,mbrubeck/servo,thiagopnts/servo,eddyb/servo,KiChjang/servo,nnethercote/servo,KiChjang/servo,SimonSapin/servo,rnestler/servo,notriddle/servo,nnethercote/servo,CJ8664/servo,jimberlage/servo,rnestler/servo,anthgur/servo,emilio/servo,mbrubeck/servo,emilio/servo,mbrubeck/servo,paulrouget/servo,dsandeephegde/servo,sadmansk/servo,paulrouget/servo,pyfisch/servo,larsbergstrom/servo,pyfisch/servo,pyfisch/servo,saneyuki/servo,saneyuki/servo,paulrouget/servo,jimberlage/servo,szeged/servo,larsbergstrom/servo,sadmansk/servo,mattnenterprise/servo,nnethercote/servo,dati91/servo,CJ8664/servo,rnestler/servo,splav/servo,dsandeephegde/servo,splav/servo,DominoTree/servo,nnethercote/servo,peterjoel/servo,CJ8664/servo,dsandeephegde/servo,SimonSapin/servo,anthgur/servo,szeged/servo,mattnenterprise/servo,larsbergstrom/servo,mattnenterprise/servo,emilio/servo,larsbergstrom/servo,rnestler/servo,thiagopnts/servo,fiji-flo/servo,DominoTree/servo,nnethercote/servo,upsuper/servo,szeged/servo,upsuper/servo,cbrewster/servo,paulrouget/servo,larsbergstrom/servo,notriddle/servo,thiagopnts/servo,dati91/servo,szeged/servo,upsuper/servo,pyfisch/servo,szeged/servo,canaltinova/servo,dati91/servo,mattnenterprise/servo,eddyb/servo,emilio/servo,szeged/servo,pyfisch/servo,CJ8664/servo,notriddle/servo,nnethercote/servo,nnethercote/servo,eddyb/servo,cbrewster/servo,pyfisch/servo,thiagopnts/servo,jimberlage/servo,ConnorGBrewster/servo,mattnenterprise/servo,notriddle/servo,fiji-flo/servo,mattnenterprise/servo,emilio/servo,KiChjang/servo,splav/servo,fiji-flo/servo,upsuper/servo,dsandeephegde/servo,szeged/servo,paulrouget/servo,avadacatavra/servo,dsandeephegde/servo,DominoTree/servo,splav/servo,eddyb/servo,cbrewster/servo,rnestler/servo,saneyuki/servo,mattnenterprise/servo,larsbergstrom/servo,sadmansk/servo,saneyuki/servo,KiChjang/servo,mbrubeck/servo,mbrubeck/servo,CJ8664/servo,avadacatavra/servo,mbrubeck/servo,SimonSapin/servo,rnestler/servo,mbrubeck/servo,szeged/servo,larsbergstrom/servo,thiagopnts/servo,notriddle/servo,peterjoel/servo,splav/servo,fiji-flo/servo,SimonSapin/servo,ConnorGBrewster/servo,dati91/servo,peterjoel/servo,SimonSapin/servo,paulrouget/servo,dati91/servo,nnethercote/servo,rnestler/servo,ConnorGBrewster/servo,pyfisch/servo,cbrewster/servo,canaltinova/servo,DominoTree/servo,eddyb/servo,dati91/servo,thiagopnts/servo,saneyuki/servo,larsbergstrom/servo,upsuper/servo,upsuper/servo,avadacatavra/servo,sadmansk/servo,anthgur/servo,cbrewster/servo,paulrouget/servo,ConnorGBrewster/servo,avadacatavra/servo,szeged/servo,canaltinova/servo,peterjoel/servo,upsuper/servo,jimberlage/servo,DominoTree/servo,thiagopnts/servo,emilio/servo,DominoTree/servo,SimonSapin/servo,larsbergstrom/servo,sadmansk/servo,cbrewster/servo,peterjoel/servo,peterjoel/servo,notriddle/servo,cbrewster/servo,rnestler/servo,CJ8664/servo,ConnorGBrewster/servo,anthgur/servo,ConnorGBrewster/servo,eddyb/servo,jimberlage/servo,CJ8664/servo,dsandeephegde/servo,mbrubeck/servo,jimberlage/servo,canaltinova/servo,KiChjang/servo,canaltinova/servo,thiagopnts/servo,paulrouget/servo,avadacatavra/servo,KiChjang/servo,anthgur/servo,emilio/servo,pyfisch/servo,DominoTree/servo,splav/servo,DominoTree/servo,CJ8664/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo,ConnorGBrewster/servo,eddyb/servo,saneyuki/servo,avadacatavra/servo,dati91/servo,splav/servo,upsuper/servo,paulrouget/servo,anthgur/servo,peterjoel/servo,peterjoel/servo,nnethercote/servo,splav/servo,saneyuki/servo,saneyuki/servo,splav/servo,notriddle/servo,sadmansk/servo,dsandeephegde/servo,sadmansk/servo,peterjoel/servo,KiChjang/servo,fiji-flo/servo,saneyuki/servo,peterjoel/servo,paulrouget/servo,avadacatavra/servo,notriddle/servo,splav/servo,emilio/servo,szeged/servo,dsandeephegde/servo,DominoTree/servo,canaltinova/servo,pyfisch/servo,nnethercote/servo,jimberlage/servo,fiji-flo/servo,anthgur/servo,fiji-flo/servo,mattnenterprise/servo,jimberlage/servo,sadmansk/servo,canaltinova/servo,avadacatavra/servo,saneyuki/servo,notriddle/servo,eddyb/servo,cbrewster/servo,canaltinova/servo,fiji-flo/servo,larsbergstrom/servo