lang/rust: remap debuginfo

Prior to this update, when the rust compiler generates warnings
or errors that refer to source code for the standard library, its
messaging use the build time path, rather than the installed path.

It's probably more useful for most users of rust to not have to
mentally rename files to find the files in question.

PR: 278133
Differential Revision: https://reviews.freebsd.org/D44547