test: don't try to find BUILD_DIR when NO_BUILD is set

NO_BUILD=1 indicates that we want to test systemd from the local system and not
the one from the local build. Hence there should be no need to call
find-build-dir.sh when NO_BUID=1 especially since it's likely that the script
will fail to find a local build in this case.

This avoids find-build-dir.sh to emit 'Specify build directory with $BUILD_DIR'
message when NO_BUILD=1 and no local build can be found.

This introduces a behavior change though: systemd from the local system will
always be preferred when NO_BUILD=1 even if a local build can be found.