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 }}

.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: |

.github/workflows/nightly-build.yml

@@ -50,7 +50,7 @@ jobs:
   # test/vm-image/nightly-plan decides, from what is fetched here.
   plan:
     name: Plan
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 5
     outputs:
       channels: ${{ steps.plan.outputs.channels }}
@@ -229,7 +229,7 @@ jobs:
       fail-fast: false
       matrix:
         include: ${{ fromJSON(needs.plan.outputs.channels) }}
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 45
     environment: nightly
     # actions: read is for finding and fetching the build job's artifact.
@@ -258,8 +258,12 @@ jobs:
       - name: Fetch the release from the build job
         env:
           GH_TOKEN: ${{ github.token }}
+          # Not /tmp: on Ubuntu 26.04 it is held in memory, and the zip is
+          # gigabytes.
+          TMPDIR: ${{ runner.temp }}
         run: |
-          sudo apt-get update -qq && sudo apt-get install -y -qq aria2 unzip >/dev/null
+          df -h "$TMPDIR" .
+          command -v aria2c unzip >/dev/null || { sudo apt-get update -qq && sudo apt-get install -y -qq aria2 unzip >/dev/null; }
           ./test/vm-image/nightly-fetch "$GITHUB_REPOSITORY" "$GITHUB_RUN_ID" "nightly-$CHANNEL-$GITHUB_RUN_ID" out
 
       # A current rclone, by version and checksum. Ubuntu's is 1.60 from 2022:

.github/workflows/unit-test.yml

@@ -15,7 +15,7 @@ permissions:
 
 jobs:
   unit:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1