Make registry gating dynamic

Created by: philipp-spiess

Follow up after #40085 (closed) c.f. https://github.com/sourcegraph/sourcegraph/pull/40473/files#diff-e38dfc4ac8d0e3f41b54d78100726d7c01914f2fda0c70849d87b5fa22201013

/cc @muratsu @jjinnii @ryankscott