oeqa/selftest: Ensure buildtools in environment variables isn't replaced
This avoids the seeing broken replacements like:
oe-selftest-centos/build/build-st-926tools/sysroots/x86_64-pokysdk-linux/etc/ssl/certs/ca-certificates.crt
which understandably break builds.
(From OE-Core rev:
04ee0e8b95cd8ed890374e0007f976684206b630)
(Cherry-picked from
f930e2cadb9ee69759720b6c49aeeb6dd43a7edd but adjusted for thud)
(From OE-Core rev:
b51a8c390714357713e91d828708c85cf5f68971)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
