Always compile rustdoc with debug logging enabled when `download-rustc` is set

Previously, logging at DEBUG or below would always be silenced, because
rustc compiles tracing with the `static_max_level_info` feature. That
makes sense for release artifacts, but not for developing rustdoc.

Instead, this compiles two different versions of tracing: one in the
release artifacts, distributed in the sysroot, and a new version
compiled by rustdoc. Since `rustc_driver` is always linked to the
version of sysroot, this copy/pastes `init_env_logging` into rustdoc.

The builds the second version of tracing unconditionally; see the code
for details on why.

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