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.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>