Upgrade test: started by hand only, and two checks that prove what they say

The workflow no longer runs for pull requests. It is started by hand,
and still posts to the pull request of the branch it was started on.
The fork fallback and the fork guard go with the trigger.

Two checks could pass without the thing they named being true:

The wallpaper check asked only whether the file the link points at
exists, so an update that swapped the user's picture for another would
pass. It now compares before and after: the same picture, whatever
format it is kept in, and present.

The failed-units check read an empty answer as nothing failed, which is
also what a service manager that cannot be asked gives. A query that
fails is now recorded as that, and fails the check.

.github/workflows/upgrade-test.yml

@@ -5,8 +5,7 @@ name: Upgrade test
 # checks what came back. test/upgrade-test does the work and says what it
 # checks.
 #
-# Started by hand: from the latest release to edge unless told otherwise. It
-# also runs for a pull request that changes the test itself.
+# Started by hand only: from the latest release to edge unless told otherwise.
 #
 # When there is a pull request for the branch it ran on, the result goes there
 # as one comment, updated in place on later runs, with a picture of the desktop
@@ -25,16 +24,12 @@ on:
         description: "Release to start from, like 4.0.4 (default: the latest)"
         type: string
         default: ""
-  pull_request:
-    paths:
-      - test/upgrade-test
-      - .github/workflows/upgrade-test.yml
 
 permissions:
   contents: read
 
 concurrency:
-  group: upgrade-test-${{ github.event.pull_request.number || github.ref }}
+  group: upgrade-test-${{ github.ref }}
   cancel-in-progress: true
 
 env:
@@ -43,9 +38,7 @@ env:
 jobs:
   upgrade:
     name: Install, upgrade, reboot
-    # Our own branches run on Namespace; a pull request from a fork stays on
-    # GitHub's runner.
-    runs-on: ${{ github.event.pull_request.head.repo.fork && 'ubuntu-26.04' || 'namespace-profile-linux-amd64-16x32' }}
+    runs-on: namespace-profile-linux-amd64-16x32
     timeout-minutes: 60
     outputs:
       from: ${{ steps.release.outputs.version }}
@@ -134,7 +127,7 @@ jobs:
   report:
     name: Report
     needs: upgrade
-    if: ${{ !cancelled() && !github.event.pull_request.head.repo.fork }}
+    if: ${{ !cancelled() }}
     runs-on: ubuntu-26.04
     timeout-minutes: 10
     permissions:
@@ -143,8 +136,7 @@ jobs:
     env:
       GH_TOKEN: ${{ github.token }}
       FROM: ${{ needs.upgrade.outputs.from }}
-      PR: ${{ github.event.pull_request.number }}
-      BRANCH: ${{ github.head_ref || github.ref_name }}
+      BRANCH: ${{ github.ref_name }}
       SHOTS_BRANCH: upgrade-test-screenshots
     steps:
       # No evidence at all still gets a comment saying so.
@@ -212,9 +204,8 @@ jobs:
           } >comment.md
           cat comment.md >>"$GITHUB_STEP_SUMMARY"
 
-          # A run started by hand has no pull request of its own: use the open
-          # one for its branch, if there is one.
-          pr=${PR:-$(gh pr list -R "$GITHUB_REPOSITORY" --head "$BRANCH" --state open --json number --jq '.[0].number // empty')}
+          # The open pull request for the branch this was started on, if any.
+          pr=$(gh pr list -R "$GITHUB_REPOSITORY" --head "$BRANCH" --state open --json number --jq '.[0].number // empty')
           [ -n "$pr" ] || { echo "No pull request for $BRANCH: the result is on this run's page."; exit 0; }
           existing=$(gh api "repos/$GITHUB_REPOSITORY/issues/$pr/comments" --paginate \
             --jq "[.[] | select(.user.login == \"github-actions[bot]\" and (.body | startswith(\"$marker\")))][0].id // empty")