index-resources: update IS_WORKFLOW_CALL to default to false

When the workflow is not triggered by `workflow_call`, then the
`is-workflow-call` input sets the `IS_WORKFLOW_CALL` environment
variable to an empty string. This commit explicitly sets the
default value for `IS_WORKFLOW_CALL` to `false` so that
`./scripts/get-resource-index-ref-matrix` can run the appropriate steps.

Follow up to #865.

.github/workflows/index-resources.yml

@@ -43,7 +43,7 @@ jobs:
         name: Create ref matrix
         env:
           HEROKU_TOKEN: ${{ secrets.HEROKU_TOKEN_READ_PROTECTED }}
-          IS_WORKFLOW_CALL: ${{ inputs.is-workflow-call }}
+          IS_WORKFLOW_CALL: ${{ inputs.is-workflow-call || false }}
         run: |
           echo "ref-matrix=$(./scripts/get-resource-index-ref-matrix)" | tee -a "$GITHUB_OUTPUT"