Use different logical material for signing keys to avoid fan-in issues

We don't need/want fan-in on the signing keys, so let's split them. Now that the
test drive installers are part of the same pipeline as the main signing, this will cause fan-in issues.

Author: chadlwilson

ci.gocd.yml

@@ -21,7 +21,7 @@ pipelines:
           - "**/*.*"
           - "**/*"
       signing-keys:
-        git: https://git.gocd.io/git/gocd/signing-keys
+        git: https://github.com/gocd/signing-keys
         destination: signing-keys
         ignore:
           - "**/*.*"