ci: install rsync for pgxntool's test-build feature

test/build/zzz_build.sql (moved there in the previous commit) enables
pgxntool's test-build feature, whose run-test-build.sh syncs
test/build/*.sql into test/build/sql/ via rsync -- not present in the
pgxn/pgxn-tools image, causing every PG-matrix leg to fail with
"rsync: command not found".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: jnasbyupgrade

.github/workflows/ci.yml

@@ -111,6 +111,11 @@ jobs:
         run: pg-start ${{ matrix.pg }}
       - name: Check out the repo
         uses: actions/checkout@v4
+      # pgxntool's test-build feature (test/build/zzz_build.sql) syncs
+      # test/build/*.sql into test/build/sql/ via rsync (run-test-build.sh),
+      # which the pgxn/pgxn-tools image doesn't ship.
+      - name: Install rsync
+        run: apt-get install -y rsync
       - name: Test on PostgreSQL ${{ matrix.pg }}
         # `make test` alone never fails this step: pgxntool/base.mk (as
         # vendored here, 2.2.0) marks its underlying installcheck .IGNORE,