Auto merge of #21490 - servo:jdm-patch-22, r=KiChjang
Auto merge of #21490 - servo:jdm-patch-22, r=KiChjang

appveyor: Append to LIB variable instead of overwriting it.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21489
- [x] There are tests for these changes

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paulrouget/servo,sadmansk/servo,notriddle/servo,nnethercote/servo,jimberlage/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo,emilio/servo,jimberlage/servo,paulrouget/servo,pyfisch/servo,paulrouget/servo,pyfisch/servo,DominoTree/servo,DominoTree/servo,emilio/servo,splav/servo,saneyuki/servo,larsbergstrom/servo,sadmansk/servo,peterjoel/servo,szeged/servo,jimberlage/servo,nnethercote/servo,notriddle/servo,DominoTree/servo,SimonSapin/servo,splav/servo,paulrouget/servo,saneyuki/servo,SimonSapin/servo,KiChjang/servo,SimonSapin/servo,DominoTree/servo,saneyuki/servo,notriddle/servo,paulrouget/servo,peterjoel/servo,pyfisch/servo,SimonSapin/servo,jimberlage/servo,sadmansk/servo,paulrouget/servo,emilio/servo,szeged/servo,peterjoel/servo,nnethercote/servo,KiChjang/servo,emilio/servo,splav/servo,larsbergstrom/servo,peterjoel/servo,splav/servo,saneyuki/servo,sadmansk/servo,notriddle/servo,larsbergstrom/servo,larsbergstrom/servo,peterjoel/servo,emilio/servo,sadmansk/servo,DominoTree/servo,emilio/servo,notriddle/servo,SimonSapin/servo,peterjoel/servo,splav/servo,peterjoel/servo,SimonSapin/servo,sadmansk/servo,szeged/servo,nnethercote/servo,nnethercote/servo,splav/servo,pyfisch/servo,SimonSapin/servo,saneyuki/servo,peterjoel/servo,jimberlage/servo,nnethercote/servo,larsbergstrom/servo,szeged/servo,splav/servo,saneyuki/servo,szeged/servo,splav/servo,jimberlage/servo,pyfisch/servo,KiChjang/servo,saneyuki/servo,DominoTree/servo,szeged/servo,notriddle/servo,larsbergstrom/servo,paulrouget/servo,jimberlage/servo,emilio/servo,notriddle/servo,DominoTree/servo,pyfisch/servo,paulrouget/servo,notriddle/servo,larsbergstrom/servo,nnethercote/servo,szeged/servo,peterjoel/servo,KiChjang/servo,pyfisch/servo,larsbergstrom/servo,splav/servo,KiChjang/servo,peterjoel/servo,emilio/servo,jimberlage/servo,notriddle/servo,DominoTree/servo,KiChjang/servo,KiChjang/servo,pyfisch/servo,paulrouget/servo,sadmansk/servo,szeged/servo,jimberlage/servo,saneyuki/servo,SimonSapin/servo,notriddle/servo,nnethercote/servo,emilio/servo,larsbergstrom/servo,DominoTree/servo,nnethercote/servo,saneyuki/servo,paulrouget/servo,szeged/servo,pyfisch/servo,sadmansk/servo,KiChjang/servo,szeged/servo,saneyuki/servo,nnethercote/servo,emilio/servo,larsbergstrom/servo,splav/servo,pyfisch/servo