[llvm] [lit] add %{readenv:<var>} to internal and external shells (PR #178064)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Ah, didn't realize that define would work here. There are a couple cases in `compiler-rt` where we need to actually get an environment variable and currently we have to write it to a file and then load it, but those are super rare and I didn't think it justified the additional complexity of a feature here.

https://github.com/llvm/llvm-project/pull/178064