chore(renovate): avoid pinning a digest for local actions (#471)

Renovate with a global `pinDigest: true` tries pinning a digest also for
local actions referenced via the
`owner/repo[/path]@ref` pattern.
A new digest update for those actions would be generated every time
there's a new commit on main, given that they live directly in this
repository.

Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

Author: NiccoloFei

renovate.json

@@ -36,6 +36,15 @@
         "action"
       ],
       "pinDigests": true
+    },
+    {
+      "matchDepTypes": [
+        "action"
+      ],
+      "matchPackageNames": [
+        "/^cloudnative-pg\\/postgres-containers\\/\\.github\\/actions\\//"
+      ],
+      "pinDigests": false
     }
   ]
 }