Fix syntax error – add braces, double-quote

If an if statement does not have operators in the expression, the GitHub
Action syntax allows you to omit the expression braces, but that's no
longer the case here.

Further, the GitHub Action YAML parser will trip up on the colon in the
sigle-quoted error message we are looking for, unless we put the entire
GitHub Action expression in double-quotes [1].

[1] actions/runner#1019 (comment)

Author: ryanhofdotgov

.github/workflows/pa11y.yml

@@ -44,8 +44,12 @@ jobs:
 
 </details>'
 
+      # A defect in pa11y is causing intermitent page crashes [1]. Until it is
+      # fixed, if a page crashes during a scan, do not mark the pa11y scan as
+      # failed.
+      # [1] https://github.com/pa11y/pa11y-ci/issues/128
       - name: Check for pa11y failures.
-        if: contains(steps.pa11y_output.outputs.content, 'errno 2') && !contains(steps.pa11y_output.outputs.content, 'UnhandledPromiseRejectionWarning: Error: Page crashed!')
+        if: "${{ contains(steps.pa11y_output.outputs.content, 'errno 2') && !contains(steps.pa11y_output.outputs.content, 'UnhandledPromiseRejectionWarning: Error: Page crashed!') }}"
         run: |
           echo "::error::The site is failing accessibility tests. Please review the comment in the pull request or the pa11y-ci step in the workflow for details."
           exit 1