Revert "Refer to the key in /usr/etc instead of /etc"

This reverts commit 5f3a520.

The `unlinkat` error was caused by an old cosign.pub file which at the
immutable bit set on it. Because of that, and because the filesystem was
mounted read-only, I couldn't delete that ostree deployment.

The solution was to use a live USB to unlock LUKS, mount the filesystem
directly as writeable, remove the immutable bit, then reboot and run
`ostree admin cleanup`

Since that's resolved now and since it makes more sense to refer to /etc
instead of /usr/etc this is getting reverted.

overlay-root/usr/etc/containers/policy.json

@@ -14,7 +14,7 @@
             "ghcr.io/samhclark": [
                 {
                     "type": "sigstoreSigned",
-                    "keyPath": "/usr/etc/pki/cosign/cosign.pub",
+                    "keyPath": "/etc/pki/cosign/cosign.pub",
                     "signedIdentity": {
                         "type": "matchRepository"
                     }