build: do not set CCACHE_COMPILERCHECK

build: do not set CCACHE_COMPILERCHECK

Stop overriding CCACHE_COMPILERCHECK from rules.mk and rely on
ccache's default, which hashes the compiler binary's mtime and
size.

For a local tree that is enough: the toolchain only changes when
it is rebuilt, and a rebuild updates mtime and size. Users who
want a stricter check can still set CCACHE_COMPILERCHECK in their
environment.

CI does not depend on this export either. The workflow writes a
secondary ccache.conf with

  compiler_check=string:<toolchain-commit-sha>

so the cache key is tied to the toolchain source revision. That
is both stricter than the previous "%compiler% -v -c" setting
and portable across runners, where compiler mtimes would not
match after restoring a cache archive.

Link: https://github.com/openwrt/openwrt/pull/22995
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>