[llvm] [LLVM][test][nfc] Prefixing JITDUMPDIR environment variable with env (PR #98370)

[Paul Kirth via llvm-commits]

ilovepi wrote:

Actually can we change the commit title to something like “Update test to work with internal shell”?

The current title isn’t all that informative IMO. 

https://github.com/llvm/llvm-project/pull/98370