use an external server for externalStagingServer b=396438 r=joduinn

tools/release/configs/fx-moz18-staging-bootstrap.cfg

@@ -62,7 +62,7 @@ buildTree       = MozillaTest
 stagingUser     = cltbld
 stagingServer   = staging-build-console.build.mozilla.org
 externalStagingUser     = cltbld
-externalStagingServer   = staging-build-console.build.mozilla.org
+externalStagingServer   = staging-prometheus-vm.build.mozilla.org
 # where beta updates/builds go
 ftpServer       = staging-build-console.build.mozilla.org
 # where release updates/builds go