Add 'latest' branch to all CI builds

With 3.4, the tip of the 'latest' branch will be a merge commit for
3.3-stable and the main branch, and not simply a fast-foward to an
already tested commit.

This sets up a tiny additional safety net before that merge.

(cherry picked from commit 3b13228650)

.appveyor.yml

@@ -4,6 +4,7 @@ branches:
     only:
         - ci
         - master
+        - latest
         - 3.3-stable
 skip_tags: true
 environment:

.github/workflows/build.yml

@@ -2,7 +2,7 @@ name: Build
 on:
     pull_request:
     push:
-        branches: [ ci, master, 3.3-stable ]
+        branches: [ ci, master, latest, 3.3-stable ]
 permissions:
     statuses: write
     contents: read