CI: tag: values are explicitly strings

Author: aramprice

ci/pipelines/stemcells-windows.yml

@@ -105,7 +105,7 @@ resource_types:
   type: docker-image
   source:
     repository: concourse/semver-resource
-    tag: 1.6
+    tag: "1.6"
     username: ((docker.username))
     password: ((docker.password))