Set debug-file-directory from environment variable?

Hello,

Would folks be okay with a patch to take debug file directories from an
environment variable, e.g. GDB_DEBUG_FILE_PATH ?

An example of when this is could be useful is for Guix users: the
variable could be automatically set up when spawning a Guix shell that
contains debug packages.

But I'm guessing it would be useful in other situations too.