build(woodpecker): split updating docker builder into seperate job

Closes kemitix/git-next#7

.woodpecker/build.yml

@@ -1,19 +1,5 @@
 steps:
 
-  update-builder-image:
-    when:
-      - event: cron
-    image: docker.io/woodpeckerci/plugin-docker-buildx:latest
-    settings:
-      username: kemitix
-      repo: git.kemitix.net/kemitix/git-next-builder
-      auto_tag: true
-      dockerfile: Dockerfile.builder
-      dry-run: false # push to remote repo
-      registry: git.kemitix.net
-      password:
-        from_secret: woodpecker-docker-push
-
   build:
     when:
       - event: push

.woodpecker/builder.yml

@@ -0,0 +1,15 @@
+steps:
+
+  update-builder-image:
+    when:
+      - event: cron
+    image: docker.io/woodpeckerci/plugin-docker-buildx:latest
+    settings:
+      username: kemitix
+      repo: git.kemitix.net/kemitix/git-next-builder
+      auto_tag: true
+      dockerfile: Dockerfile.builder
+      dry-run: false # push to remote repo
+      registry: git.kemitix.net
+      password:
+        from_secret: woodpecker-docker-push