CI: Lower the PostgreSQL version on Macos to 13.2

Although our custom libpq of PostgreSQL-14 is regisered per rpath, the system library is loaded instead.
The error looks like so:

dyld: lazy symbol binding failed: Symbol not found: _PQpipelineStatus
  Referenced from: /Users/runner/.rubies/ruby-head/lib/ruby/gems/3.1.0/gems/pg-1.2.3/lib/pg_ext.bundle
  Expected in: /usr/local/lib/libpq.5.dylib

.github/workflows/source-gem.yml

@@ -54,8 +54,8 @@ jobs:
             PGVER: "14"
           - os: macos
             ruby: "head"
-            PGVERSION: 14.0-1-osx
-            PGVER: "14"
+            PGVERSION: 13.2-1-osx
+            PGVER: "13"
 
     runs-on: ${{ matrix.os }}-latest
     env: