[Chapel Merge] Print a better error message when an environment v

Branch: refs/heads/main
Revision: bb37008
Author: jhh67
Link: Unavailable
Log Message:

Merge pull request #19546 from jhh67/T19063

Print a better error message when an environment variable that affects the runtime build (e.g. CHPL_LAUNCHER) is invalid, including a list of valid values.

[Reviewed by @gbtitus and @ronawho]

Modified Files:
M runtime/Makefile.help

Compare: https://github.com/chapel-lang/chapel/compare/ee4bae675214...bb370084e2e6