feat: add an env variable to toggle pipstar #2855
                
     Merged
            
            
          
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This is a flag to start leveraging of the new code paths. The Starlark
implementation has been added in 1.4 and has been reverted in the latest
release candidates. The
envvariable will be a good way to roll it out moregradually and get more testing.
For now we are switching only the
whl_libraryinternals as therequirements.txtfiles fromuvmay use*inpython_full_versionandplatform_versionthat are not yet fully supported (#2826).Main goals for this is to start using Starlark implementation so that we don't
have any hidden variables. What is more, having this in Starlark is the most
maintainable long-term solution for supporting cross-platform builds.
Work towards #260