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/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: