Fix fallback input for release action for real

Turns out `'master'` probably worked all along as a fallback, the
problem is that I was using `'master '`, with a trailing space, which
was not a branch git managed to find for obvious reason, and since I
carried the error into the fully qualified reference... is still
didn't work.

And manual triggers didn't have the issue because the tag was
`required`, so I'd have to input the tag by hand every time, and the
fallback value would be bypassed.

- fix the fallback value
- remove the requirement on `tag`, such that it's possible to manually
  trigger the action in a default state

.github/workflows/release-builtins.yml

@@ -11,7 +11,6 @@ on:
       tag:
         description: "uap-core ref to release"
         type: string
-        required: true
       environment:
         description: "environment to release for (testpypy or pypy)"
         type: environment
@@ -30,7 +29,7 @@ jobs:
         persist-credentials: false
     - name: update core
       env:
-        TAG: ${{ inputs.tag || 'origin/master '}}
+        TAG: ${{ inputs.tag || 'origin/master' }}
       # needs to detach because we can update to a tag
       run: git -C uap-core switch --detach "$TAG"
     - name: Set up Python