check for string true rather than existence in setup-python-keyring (#41)

We saw builds breaking because `python -m pip install
"keyrings-chainguard-libraries"` was executing on custom runners which
don't have the `python` executable installed. The intent seems to be
that this step only runs when `setup-python-keyring` is overriden to
`true`, and not by default, but the current implementation will always
run it (the `if` statement as currently written evaluates to `true`
because the variable exists). This fixes the `if` statement to only
evaluate to `true` if `setup-python-keyring` has been overridden to
`true`.

Author: steven-rand

action.yaml

@@ -200,7 +200,7 @@ runs:
           echo HTTP_AUTH="basic:${{ inputs.apk-host }}:user:$(chainctl auth token --audience ${{ inputs.apk-host }})" >> $GITHUB_ENV
         fi
 
-    - if: ${{ inputs.setup-python-keyring }}
+    - if: ${{ inputs.setup-python-keyring == 'true' }}
       name: Install Python keyring package
       shell: bash
       run: |