Add permissions tofiles-changed jobs (#36142)

Followup to https://github.com/go-gitea/gitea/pull/36140.
`files-changed` is a job that imports another workflow via `uses`
statement but CodeQL still complains about lack of permissions on these
jobs, so add it. This will fix the remaining [3 CodeQL
issues](https://github.com/go-gitea/gitea/security/code-scanning?query=is%3Aopen+branch%3Amain+permissions).

.github/workflows/pull-compliance.yml

@@ -10,6 +10,8 @@ concurrency:
 jobs:
   files-changed:
     uses: ./.github/workflows/files-changed.yml
+    permissions:
+      contents: read
 
   lint-backend:
     if: needs.files-changed.outputs.backend == 'true' || needs.files-changed.outputs.actions == 'true'

.github/workflows/pull-db-tests.yml

@@ -10,6 +10,8 @@ concurrency:
 jobs:
   files-changed:
     uses: ./.github/workflows/files-changed.yml
+    permissions:
+      contents: read
 
   test-pgsql:
     if: needs.files-changed.outputs.backend == 'true' || needs.files-changed.outputs.actions == 'true'

.github/workflows/pull-docker-dryrun.yml

@@ -10,6 +10,8 @@ concurrency:
 jobs:
   files-changed:
     uses: ./.github/workflows/files-changed.yml
+    permissions:
+      contents: read
 
   container:
     if: needs.files-changed.outputs.docker == 'true' || needs.files-changed.outputs.actions == 'true'