[compiler-rt] [llvm-lit] Change tests to remove the use of `unset` command in lit internal shell (PR #104880)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Naively, this looks fine to me. However, I don't know the compiler-rt tests at all, so this needs a more knowledgeable person to give it a once over. Perhaps worth mentioning in the description the differences between `env -u` and `unset`.

https://github.com/llvm/llvm-project/pull/104880