[ci] Run tests on pull requests

The automated tests that are run in the GitHub Actions workflow are
now as comprehensive as those that are run manually.  Run tests on
pull requests as well as pushes, since the results are now
meaningfully informative.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

.github/workflows/build.yml

@@ -1,6 +1,8 @@
 name: Build
 
-on: push
+on:
+  push:
+  pull_request:
 
 env:
   MAKEFLAGS: "-j4 GITVERSION=${{ github.sha }}"