selftest/signing: Ensure build path relocation is safe

Similarly to 04ee0e8b, ensure only full
build paths are replaced in the environment to avoid breaking buildtools.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit fcd0a968)
Signed-off-by: Steve Sakoman <steve@sakoman.com>