selftest/signing: Ensure build path relocation is safe
Similarly to 04ee0e8b95cd8ed890374e0007f976684206b630, ensure only full
build paths are replaced in the environment to avoid breaking buildtools.
(From OE-Core rev: be07d93a4f59d4563f2d064be1997b39f05e9f0e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>