build-deps.txt: add shellcheck

CI was updated[0] to use coreos-assembler as a buildroot instead of the fcos-buildroot container and
the openshift/os `validate` job[1] fails because shellcheck isn't installed in coreos-assembler. It
cannot be installed using `sudo` when running on OpenShift so it needs to be installed directly in
coreos-assembler.

[0] openshift/release@68a7999
[1] https://github.com/openshift/os/blob/ce4da959eb3fc9477a0a96b69e6efd084a69b9ac/ci/prow-entrypoint.sh#L162

src/build-deps.txt

@@ -2,6 +2,8 @@
 time
 # Used by mantle
 golang
+# Used for syntax validation of manifests
+shellcheck
 # Use tmux for developer multi-arch builder access UX 
 #https://github.com/coreos/fedora-coreos-pipeline/issues/803
 tmux