In zerocopy-derive UI tests, detect whether we're building with RUSTFLAGS='-Wwarnings'

[joshlf]

This is how we run zerocopy-derive's UI tests in CI, and so if a developer locally runs UI tests treating warnings as errors, they will generate the wrong .stderr files, and CI will fail.

If RUSTFLAGS='-Wwarnings' is absent the test harness should eagerly fail with a single descriptive error, rather than proceed to run UI tests (which will produce lots of errors).

For those interested in tackling this, the file to look in is: https://github.com/google/zerocopy/blob/main/zerocopy-derive/tests/trybuild.rs