Fix assumptions about go binary being in the PATH in CI tests

Getting the GOOS/GOARCH here is not necessary for every single target, especially the ones that start test fixtures and don't need Go at all, which creates some unnecessary noise in the logs when make is invoked without a go binary in the PATH (this is the case on CI hosts). It would be good to not shell out in cases where go is irrelevant. Alternatively, hard-code the known path to the go binary (not great, but maybe it's fine).