oeqa/selftest: Only rewrite envvars paths that absolutely point to builddir

When building the new selftest builddir, paths in environment variables
are rewritten to point to the new buildir, but users can have
environment variables that point outside of the build dir using
relative paths from builddir. We must not rewrite those.

Check this by verifying that the absolute path still contains the
builddir.

Fixes [YOCTO #15241]

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>