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.

test/vm-image/nightly-fetch

@@ -8,11 +8,14 @@
 # release would outlast the job. An artifact is one zip at a signed address,
 # so this asks the API for that address and takes it over 16 connections.
 #
+# The zip goes to TMPDIR and is deleted once unpacked, so there has to be room
+# for the artifact twice: point TMPDIR at a disk, not at a /tmp held in memory.
+#
 # Needs gh (with GH_TOKEN allowed to read the run's artifacts), curl, aria2c
 # and unzip. A run started again keeps the artifacts of its earlier attempts
 # under the same name: the newest is the one fetched.
 set -euo pipefail
-[[ $# -eq 4 ]] || { sed -n '2,13p' "$0" >&2; exit 2; }
+[[ $# -eq 4 ]] || { sed -n '2,16p' "$0" >&2; exit 2; }
 repo=$1 run=$2 name=$3 out=$4
 fail() { echo "nightly-fetch: $*" >&2; exit 1; }
 
@@ -23,6 +26,11 @@ size=$(gh api "repos/$repo/actions/artifacts/$id" --jq .size_in_bytes)
 echo "nightly-fetch: artifact $id, $((size / 1048576)) MiB"
 
 zip=$(mktemp -u "${TMPDIR:-/tmp}/nightly-fetch.XXXXXX.zip")
+mkdir -p "$out"
+for place in "$(dirname "$zip")" "$out"; do
+  free=$(( $(df --output=avail -B1 "$place" | tail -n 1) ))
+  (( free > size )) || fail "$place has $((free / 1048576)) MiB free, the artifact is $((size / 1048576)) MiB"
+done
 trap 'rm -f "$zip" "$zip.aria2"' EXIT
 started=$(date +%s)
 # The address is signed for a few minutes. A try that fails asks for a new one
@@ -44,6 +52,5 @@ done
 seconds=$(( $(date +%s) - started ))
 echo "nightly-fetch: downloaded in $seconds s, $(( size / 1048576 / (seconds > 0 ? seconds : 1) )) MiB/s"
 
-mkdir -p "$out"
 unzip -q "$zip" -d "$out"
 echo "nightly-fetch: $(find "$out" -type f | wc -l) files in $out"