Fix rustc_tools_util's version.host_compiler release channel, expose the rustc version, and add tests

[ignatz]

changelog: Fix rustc_tools_util's version.host_compiler release channel, expose the rustc version, and add tests.

Previously the host_compiler would be set to "nighly" on the stable channel. Generally, the field felt a bit neglected neither being printed not tested.

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[ignatz]

FWIW, I would also happily replace

host_compiler = "stable" | "nightly" | "beta" | undefined;

with the verbose version string, e.g. "rustc 1.84.0 (9fc6b4312 2025-01-07)"

The former is trivial to derive. Merely didn't do it in case someone depends on the current behavior, though unlikely since it was broken. Maybe one could just release a 0.5.0?