.github/workflows: purge Cloudflare cache on deployment

We use Cloudflare in front of the git-scm.com website in order to cache
traffic that would otherwise be going directly to GitHub Pages. However,
this cache needs to be purged whenever the actual site is updated,
otherwise clients will have to wait out Cloudflare's and their own
browser's cache TTL until they see the new content.

Usually this is just fine: for high priority deployments where the new
content should be reflected immediately, someone can manually purge the
cache by logging into Cloudflare and clicking on the "Purge everything"
button.

But sometimes folks aren't around to log into Cloudflare, don't have
permissions to, etc. Let's remove the need to have anyone log into
Cloudflare and instead purge the cache on deployment so that the site is
more up-to-date more quickly upon deployment.

(Note that this only purges the cache for git-scm.com, not git-scm.org.
The former receives the vast majority of traffic, and when purging
manually I almost always skipped git-scm.org without complaints. Let's
avoid having two API keys and instead let git-scm.org lag slightly
behind git-scm.com after a deployment).

To generate the API key, I logged into Cloudflare and created an API
token[1] that has permissions limited to "Zone.Cache Purge" for all
zones in the Git project's Cloudflare account. The token is stored as a
repository secret[2].

[1]: https://developers.cloudflare.com/fundamentals/api/get-started/create-token/
[2]: https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions

Signed-off-by: Taylor Blau <me@ttaylorr.com>

.github/workflows/deploy.yml

@@ -21,3 +21,8 @@ jobs:
       - name: deploy to GitHub Pages
         id: deploy
         uses: ./.github/actions/deploy-to-github-pages
+      - name: Purge Cloudflare cache
+        uses: jakejarvis/cloudflare-purge-action@v0.3.0
+        env:
+          CLOUDFLARE_ZONE: ${{ secrets.CLOUDFLARE_ZONE }}
+          CLOUDFLARE_TOKEN: ${{ secrets.CLOUDFLARE_TOKEN }}