[github workflow] Resolve token-permissions security alerts (#5627)

Author: sealesj

.github/workflows/mirror.yml

@@ -7,6 +7,9 @@ on:
   push:
     branches:
       - 'main'
+      
+# Declare default permissions as read only.
+permissions: read-all
 
 jobs:
   mirror_job:

.github/workflows/pull_request_label.yml

@@ -12,6 +12,9 @@ on:
   pull_request_target:
     types: [opened, synchronize, reopened, closed]
 
+# Declare default permissions as read only.
+permissions: read-all
+
 jobs:
   label:
     permissions:

.github/workflows/release.yml

@@ -4,6 +4,9 @@ on:
     branches:
       - main
 
+# Declare default permissions as read only.
+permissions: read-all
+
 jobs:
   release:
     if: github.repository_owner == 'flutter'