Fix the cmdlineargs tests on Buildkite

[DilumAluthge]

This change is necessary to get the cmdlineargs tests passing on Buildkite.

Ref #41977

[vtjnash]

Is buildkite forcing threads to be enabled? If so, that is a buildkite bug.

[DilumAluthge]

Is buildkite forcing threads to be enabled? If so, that is a buildkite bug.

In our Buildkite setup, we have it set up to run with JULIA_NUM_THREADS=16.

But this isn't actually specific to Buildkite. If you do export JULIA_NUM_THREADS=4 locally, and then run the cmdlineargs tests, they will fail.

I think we want these tests to pass regardless of whether or not the user has set the JULIA_NUM_THREADS environment variable, right?

[DilumAluthge]

An alternate approach would be to run these particular tests inside a withenv that sets JULIA_NUM_THREADS to nothing.

[DilumAluthge]

The other alternative is to fix the bug described in your MWE in #41977 (comment).

I have no idea how to fix that bug 😭, which is why I figured I'd patch the tests instead.

[DilumAluthge]

@staticfloat @vtjnash Can I merge this?

As mentioned above, this PR is necessary to get Buildkite working.

But also, it's necessary to fix the cmdlineargs tests for any user that has e.g. export JULIA_NUM_THREADS=4 in their environment.

[staticfloat]

Is buildkite forcing threads to be enabled? If so, that is a buildkite bug.

We need a way to limit the number of threads Julia spawns with (since we run multiple runners on a giant 128-core system), so we set JULIA_NUM_THREADS as part of the environment.

I say let's merge this as-is, then revisit.