release workflow: increase timeout to 30 minutes
release workflow: increase timeout to 30 minutes

In the 1.6.7 release, we saw significantly longer execution time for
producing builds that exceeded the previous timeout of 10 minutes,
causing the workflow to fail.  After increasing to 20 minutes in the
release/1.6 branch, we continued to see one failure (which succeeded on
retry).

Increase to 30 minutes to provide additional buffer for the build to
complete.

Signed-off-by: Samuel Karp <97468ec0e303d67c50f7bb93576f42916fd517d7@google.com>

containerd/containerd,mikebrow/containerd,thaJeztah/containerd,estesp/containerd,estesp/containerd,thaJeztah/containerd,mikebrow/containerd,containerd/containerd