Default the work directory to /var/tmp

/tmp is memory-backed on some systems, and the work directory holds about
10 GiB.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

build-windows-install-disk.sh

@@ -111,7 +111,9 @@ fi
 # stat prints a file size with -f %z on macOS (BSD) and -c %s on Linux (GNU).
 if [[ "$os" == Darwin ]]; then size_args=(-f %z); else size_args=(-c %s); fi
 
-work_dir="$(mktemp -d "${TMPDIR:-/tmp}/win11-install-disk.XXXXXX")"
+# The work directory holds about 10 GiB, so it defaults to /var/tmp: /tmp is
+# memory-backed on some systems (Debian 13, Fedora, Arch).
+work_dir="$(mktemp -d "${TMPDIR:-/var/tmp}/win11-install-disk.XXXXXX")"
 mount_dir="$work_dir/iso"
 mkdir "$mount_dir"
 note "work directory: $work_dir"