Update buildcache key index when we update the package index (#19117)

This changes makes sure that when we run the pipeline job that updates
the buildcache package index on the remote mirror, we also update the
key index.  The public keys corresponding to the signing keys used to
sign the package was pushed to the mirror as a part of creating the
buildcache index, so this is just ensuring those keys are reflected
in the key index.

Also, this change makes sure the "spack buildcache update-index"
job runs even when there may have been pipeline failures, since we
would like the index always to reflect the true state of the mirror.

share/spack/spack-completion.bash

@@ -434,7 +434,7 @@ _spack_buildcache_copy() {
 }
 
 _spack_buildcache_update_index() {
-    SPACK_COMPREPLY="-h --help -d --mirror-url"
+    SPACK_COMPREPLY="-h --help -d --mirror-url -k --keys"
 }
 
 _spack_cd() {