Add querystring param to cache-bust pages URL

The nightly Lighthouse run pulls down a list of URLs to test from a
URL that is configured via a GitHub Secret. This secret value currently
points to a GitHub gist raw URL, e.g.

https://gist.githubusercontent.com/username/abcdef123456/raw/

Unfortunately, this URL doesn't always return the latest version of the
gist, apparently due to some caching on GitHub's side. To try to
bypass this caching, we can try to add a random querystring to the URLs
being fetched, e.g. ?12345678.

.github/workflows/lighthouse.yml

@@ -19,8 +19,9 @@ jobs:
       - name: Optionally fetch list of cf.gov pages to test
         env:
           TEST_PAGES_URL: ${{ secrets.TEST_PAGES_URL }}
+        # Add a random date query string to the URL to cache-bust.
         run: >
-          [[ -n "${TEST_PAGES_URL}" ]] && wget -O urls.txt "$TEST_PAGES_URL"
+          [[ -n "${TEST_PAGES_URL}" ]] && wget -O urls.txt "$TEST_PAGES_URL?$(date +%s)"
         continue-on-error: true
 
       - name: Run Lighthouse CLI against specified cf.gov pages