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.yml

@@ -25,7 +25,7 @@ jobs:
     # GitHub's runner: it is somebody else's code, and it has no business on
     # our account's machines. No cache volume either way: what one pull
     # request leaves in a cache the next run would trust.
-    runs-on: ${{ github.event.pull_request.head.repo.fork && 'ubuntu-24.04' || 'namespace-profile-linux-amd64-16x32' }}
+    runs-on: ${{ github.event.pull_request.head.repo.fork && 'ubuntu-26.04' || 'namespace-profile-linux-amd64-16x32' }}
     timeout-minutes: 75
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
@@ -43,7 +43,7 @@ jobs:
         run: NO_BOOT_OFFER=1 ./bin/omarchy-iso-make
 
       # The harness runs in an Arch container: Arch's current QEMU and OVMF
-      # (QEMU 11) instead of Ubuntu 24.04's QEMU 8.2, at the paths the
+      # (QEMU 11) instead of the runner's older one, at the paths the
       # harness expects, the same as on an Omarchy workstation.
       - name: Install unattended, then boot the installed system
         run: |