[compiler-rt] [llvm-lit] Fix error in compiler-rt tests when using lit internal shell add env (PR #102069)

[Fangrui Song via llvm-commits]

MaskRay wrote:

`VAR=a command` is so commonly used that perhaps the internal lit shell should be taught this syntax.
Then we can avoid these changes.

https://github.com/llvm/llvm-project/pull/102069