[llvm] [LIT] Fix env without subcommand causing early return in pipeline (PR #184028)

[Aaron Chen via llvm-commits]

nailo2c wrote:

Hi @jh7370, thanks for the review!

1. I wrote the PR description manually but used AI to tweak the grammar and spelling. I usually use the Why/How/What structure because I find it clear and review-friendly. Thanks for the reminder-- I will keep it shorter and more concise in future PRs. And it's my bad that I missed reading the AI policy, let me read it thoroughly and make sure to follow it.
2. Before this fix, these tests would return 0 early because they all had an `env` command in a pipeline without subcommand. (e.g. `env FOO=2 env BAR=1 | FileCheck -check-prefix=CHECK-2-VAL %s`). This caused `FileCheck` to never run, and these tests weren't failing.

https://github.com/llvm/llvm-project/pull/184028