Fix Gitea actions syntax on push events

The Github documentation is difficult to parse, but there is an
asymmetry in the API:
- for a PR event, the event object contains a `pull_request` object
  containing `base` and `head` objects
- for a push event, the `before` and `after` SHAs are immediate
  children of the event object

<https://docs.github.com/en/webhooks/webhook-events-and-payloads>

Fixes #7730.

.gitea/workflows/checkrefs.yml

@@ -4,8 +4,8 @@ on:
   - push
   - pull_request
 env:
-  BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.push.before }}
-  HEAD_SHA: ${{ github.event.pull_request.head.sha || github.event.push.after }}
+  BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.before }}
+  HEAD_SHA: ${{ github.event.pull_request.head.sha || github.event.after }}
 
 jobs:
   lfscheck:

.gitea/workflows/lint.yml

@@ -2,10 +2,11 @@
 # yamllint disable rule:line-length
 name: lint
 on:
+  - push
   - pull_request
 env:
-  BASE_SHA: ${{ github.event.pull_request.base.sha }}
-  HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+  BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.before }}
+  HEAD_SHA: ${{ github.event.pull_request.head.sha || github.event.after }}
 
 jobs:
   cppcheck: