workflows/tests: run tap_syntax job only on Linux.
workflows/tests: run tap_syntax job only on Linux.

Some food for thought as:

- the only thing that is different on macOS is the `brew readall`
  behaviour on differing macOS/Linux Formula code (and that's more
  likely to be already working and will be caught by our self-hosted
  workers)
- the Linux workers are dramatically faster than the macOS ones and this
  saves the macOS Actions workers for the cases where they are actually
  needed.

makigumo/homebrew-core,spaam/homebrew-core,stgraber/homebrew-core,cblecker/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,mahori/homebrew-core,jabenninghoff/homebrew-core,maxim-belkin/homebrew-core,lemzwerg/homebrew-core,uyjulian/homebrew-core,jvillard/homebrew-core,lembacon/homebrew-core,maxim-belkin/homebrew-core,lembacon/homebrew-core,dsXLII/homebrew-core,jabenninghoff/homebrew-core,dsXLII/homebrew-core,mbcoguno/homebrew-core,JCount/homebrew-core,mfikes/homebrew-core,zyedidia/homebrew-core,jvillard/homebrew-core,lembacon/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,jvillard/homebrew-core,spaam/homebrew-core,mbcoguno/homebrew-core,rwhogg/homebrew-core,lemzwerg/homebrew-core,nandub/homebrew-core,Moisan/homebrew-core,chrisfinazzo/homebrew-core,nbari/homebrew-core,sjackman/homebrew-core,sjackman/homebrew-core,cblecker/homebrew-core,mbcoguno/homebrew-core,Moisan/homebrew-core,nandub/homebrew-core,mfikes/homebrew-core,filcab/homebrew-core,Homebrew/homebrew-core,Linuxbrew/homebrew-core,nbari/homebrew-core,uyjulian/homebrew-core,Linuxbrew/homebrew-core,chrisfinazzo/homebrew-core,adam-moss/homebrew-core,mahori/homebrew-core,rwhogg/homebrew-core,JCount/homebrew-core,makigumo/homebrew-core,zyedidia/homebrew-core