[llvm] [llvm-lit] Print environment variables when using env without subcommand (PR #98414)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Couple more suggestions, but this basically looks good.

I'm not convinced the `# error` check not lines are needed, but I'm also not opposed to them.

https://github.com/llvm/llvm-project/pull/98414