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/unit/nightly-test.sh

@@ -181,6 +181,8 @@ STUB
   refused "an artifact that does not exist" env FETCH_IDS=" " PATH="$stubs:$PATH" TMPDIR="$tmp" FETCH_LOG="$tmp/fetch.log" "$fetch" omacom/omarchy-iso 7 nightly-edge-7 "$tmp/f3"
   refused "a download of the wrong size" env FETCH_SIZE=1 PATH="$stubs:$PATH" TMPDIR="$tmp" FETCH_LOG="$tmp/fetch.log" "$fetch" omacom/omarchy-iso 7 nightly-edge-7 "$tmp/f4"
   [[ ! -e $tmp/f4/edge ]] || fail "fetch: a download of the wrong size was unpacked"
+  refused "an artifact larger than the free space" env FETCH_SIZE=9000000000000000 PATH="$stubs:$PATH" TMPDIR="$tmp" FETCH_LOG="$tmp/fetch.log" "$fetch" omacom/omarchy-iso 7 nightly-edge-7 "$tmp/f5"
+  grep -q "MiB free" "$tmp/refused.err" || fail "fetch: too little room is not what was reported"
 else
   echo "nightly-fetch tests skipped: no zip or unzip here"
 fi