[bazel] Repair clang_headers_gen when run on macOS.

The antique version of bash (3.2.57, from 2007) which is available on
macOS cannot deal with quoted slashes in a `${x/...}`
substitution. Since only prefix-removal is required here, switch to a
`${x#...}` substitution instead.

(E.g. `src="foo/bar/baz.h"; echo ${src/"foo/bar"}` echos `bar/bar/baz.h`
instead of `/baz.h` on old bash versions).

Originally broken by 459420c3.
Fixes #63222