[clang] [Darwin][Driver][clang] Prioritise command line args over `DEFAULT_SYSROOT` (PR #115993)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

Dropped the `--sysroot=""` flag from the test.

> Changing the test itself (to e.g. locally set that env var) is a good regression test that should be added for this pr anyway.

Sorry, which env var are you referring to?

https://github.com/llvm/llvm-project/pull/115993