CI: GitHub's runners on Ubuntu 26.04

The four jobs on GitHub's own runners move from ubuntu-24.04 and
ubuntu-latest to ubuntu-26.04.

On 26.04 /tmp is held in memory, so the publish job's artifact, 11 GB for
edge, goes to the runner's temp directory on disk instead. nightly-fetch
now checks there is room before it starts and says how much is missing.

.github/workflows/install-test-comment.yml

@@ -18,7 +18,7 @@ permissions:
 jobs:
   comment:
     if: github.event.workflow_run.event == 'pull_request'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     env:
       GH_TOKEN: ${{ github.token }}
       REPO: ${{ github.repository }}