Fix runner label issues (#739)

### Description of the change

This addresses 2 bugs regarding configured runner labels:
- `Cannot find: node in PATH` due to a default label that matches the runner host, not the docker-in-docker environment
- After changing the runner config, users have to manually replace the StatefulSet Pod of the runner

### Benefits

- Out-of-the-box functioning node environment
- Auto-updated runner config on changes in values

### Applicable issues

- fixes https://gitea.com/gitea/helm-chart/issues/728
- fixes https://gitea.com/gitea/helm-chart/issues/725

### Additional information

With removing the default labels from `values.yaml`, the act runner defaults will be used for new runners which fixes the "missing node" issue.
I had issues replacing the labels once the Chart defaults were used, so users might have to replace their runner manually if this is an issue.

### Checklist

- [x] Templating unittests are added

Signed-off-by: justusbunsi <sk.bunsenbrenner@gmail.com>

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/739
Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-committed-by: justusbunsi <sk.bunsenbrenner@gmail.com>

templates/gitea/act_runner/statefulset.yaml

@@ -21,6 +21,8 @@ spec:
       {{- include "gitea.selectorLabels.actRunner" . | nindent 6 }}
   template:
     metadata:
+      annotations:
+        checksum/config: {{ include (print $.Template.BasePath "/gitea/act_runner/config-act-runner.yaml") . | sha256sum }}
       labels:
         {{- include "gitea.labels.actRunner" . | nindent 8 }}
         {{- with .Values.actions.statefulset.labels }}