feat(ci): switch to our custom runners (default)

These runners are cheaper and more reliable than the github default
runners.

Author: potyl

.github/workflows/merge-main.yaml

@@ -11,7 +11,7 @@ on:
 jobs:
   build-push:
     name: Build and push application image
-    runs-on: ubuntu-latest
+    runs-on: default
     permissions:
       id-token: write
       contents: read

.github/workflows/release-tag.yaml

@@ -10,7 +10,7 @@ jobs:
 
   re-tag:
     name: Re-tag image
-    runs-on: ubuntu-latest
+    runs-on: default
     permissions:
       id-token: write
       contents: read