Use the correct libbuildboxcommon.a

The buildbox-common image used in this CI has libbuildboxcommon.a installed to /usr/local/lib/. This is the lib that gets used by the builds of buildbox-run-hosttools and buildbox-worker in every repository's e2e.

Unfortunately, the builds of buildbox-run-hosttools and buildbox-worker in the e2e test in MRs to the buildbox-common repository also use this libbuildboxcommon.a, which will always lag behind the build of libbuildboxcommon.a in the MR itself.

This change forces buildbox-worker to look at $BUILDBOX_E2E_ROOT before looking at /usr/local/lib/.