Allow creation of symlinks for Goma via an environment variable. (#23765)
Allow creation of symlinks for Goma via an environment variable. (#23765)

Setting `FLUTTER_GOMA_CREATE_XCODE_SYMLINKS=1` as an environment variable should
achieve the same result as manually specifying the `--xcode-symlinks` flag
(which I often forget). This is in keeping with the pattern of setting the
`GOMA_DIR` environment variable. The variable has a the `FLUTTER_` prefix
because environment variable with the `GOMA_` prefix are reserved for use by
Goma and variable unknown to Goma will cause a fatal error.
chinmaygarde/flutter_engine,flutter/engine,jamesr/sky_engine,jason-simmons/sky_engine,jamesr/sky_engine,flutter/engine,jamesr/sky_engine,rmacnak-google/engine,jamesr/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,jason-simmons/flutter_engine,rmacnak-google/engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,Hixie/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine,devoncarew/engine,Hixie/sky_engine,chinmaygarde/sky_engine,chinmaygarde/sky_engine,Hixie/sky_engine,jamesr/sky_engine,jason-simmons/sky_engine,jason-simmons/sky_engine,jason-simmons/flutter_engine,Hixie/sky_engine,Hixie/sky_engine,flutter/engine,jason-simmons/sky_engine,devoncarew/engine,aam/engine,jamesr/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,flutter/engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,rmacnak-google/engine,chinmaygarde/sky_engine,flutter/engine,chinmaygarde/sky_engine,devoncarew/sky_engine,aam/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,devoncarew/engine,jamesr/flutter_engine,flutter/engine,aam/engine,devoncarew/engine,chinmaygarde/sky_engine,rmacnak-google/engine,jamesr/flutter_engine,jamesr/sky_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,aam/engine,jason-simmons/sky_engine,aam/engine,rmacnak-google/engine,devoncarew/sky_engine,jason-simmons/flutter_engine,aam/engine,jason-simmons/flutter_engine,flutter/engine,jamesr/flutter_engine,devoncarew/sky_engine,Hixie/sky_engine,devoncarew/engine,devoncarew/sky_engine,aam/engine,devoncarew/engine,jamesr/flutter_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,jamesr/flutter_engine,devoncarew/engine,rmacnak-google/engine,aam/engine,devoncarew/sky_engine,chinmaygarde/sky_engine,flutter/engine