Fix shell script quoting, clean up build scripts

Fix quoting so that it works with arbitrary path names (e.g. containing
spaces.)  Make hack/config-go.sh non-executable since it is meant for
sourcing and not as a standalone.

Tested:
- Checked out the tree to a directory with spaces, called the
  build script from outside the tree, confirmed it worked as expected.
- Confirmed binaries work by running them with -version.
- Ran hack/build-go.sh cmd/kubecfg and confirmed only kubecfg was built.
- Ran hack/build-go.sh cmd/integration and confirmed it was built.
- Checked it out on a Mac and confirmed that the build script works.
- Confirmed that hack/test-go.sh and hack/test-cmd.sh work.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>