build: Remove OUT_DIR probe artifact

[zertosh]

Remove the can_compile() probe artifact from OUT_DIR at the end of the build script, so OUT_DIR ends up empty after the script runs.

Where the probe output goes has gone back and forth:

• Don't store build check rmeta #1200 (2024): moved the output out of OUT_DIR (-o -) because the emitted .rmeta is not byte-reproducible and destabilizes cache keys in build systems that capture OUT_DIR as a build output (e.g. Bazel).
• build: Use temporary file for can_compile #1497 (2025): -o - made rustc create a temporary metadata directory in the current working directory, which broke sandboxed builds (ChromeOS Portage), so the output moved to std::env::temp_dir().
• build: Use OUT_DIR for test compilation output #1526 (2025): writing to the global temp dir left stray files on the host, so the output moved back into OUT_DIR, which reintroduced the nondeterminism that Don't store build check rmeta #1200 fixed.

Writing the probe to OUT_DIR and deleting it before the script exits satisfies all three constraints: no nondeterministic content left in OUT_DIR, no writes to the working directory, and no clutter in the system temp dir.