ci: temporarily downgrade actions/checkout from 7 to 6 (#2379)

Temporarily downgrade actions/checkout from 7 to 6 until resolving
pull_request_target uses blocked from the otherwise pulled upstream
commit [1] ("block checking out fork pr for pull_request_target and
workflow_run (#2454)").

[1]: f9e715a95f

Reverts: a6a493119e ("ci: bump actions/checkout from 6 to 7")
Link: https://github.com/nix-community/stylix/pull/2379

Reviewed-by: 0xda157 <da157@voidq.com>

.github/workflows/backport.yml

@@ -40,7 +40,7 @@ jobs:
           permission-contents: write
           permission-pull-requests: write
 
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@v6
         with:
           ref: ${{ github.event.pull_request.head.sha }}
           token: ${{ steps.app-token.outputs.token }}

.github/workflows/doc.yml

@@ -28,7 +28,7 @@ jobs:
       url: ${{ steps.deployment.outputs.page_url }}
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
 

.github/workflows/request-reviewers.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-24.04
     if: vars.APP_ID
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@v6
         with:
           ref: ${{ github.base_ref }}
 

.github/workflows/update-flake.yml

@@ -26,7 +26,7 @@ jobs:
           permission-contents: write
           permission-pull-requests: write
 
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@v6
         with:
           ref: ${{ matrix.branch }}
           token: ${{ steps.generate-token.outputs.token }}