Upgrade test: started by hand only, and two checks that prove what they say

The workflow no longer runs for pull requests. It is started by hand,
and still posts to the pull request of the branch it was started on.
The fork fallback and the fork guard go with the trigger.

Two checks could pass without the thing they named being true:

The wallpaper check asked only whether the file the link points at
exists, so an update that swapped the user's picture for another would
pass. It now compares before and after: the same picture, whatever
format it is kept in, and present.

The failed-units check read an empty answer as nothing failed, which is
also what a service manager that cannot be asked gives. A query that
fails is now recorded as that, and fails the check.

test/upgrade-test

@@ -101,8 +101,11 @@ facts() { # file
     echo "systemd=$(systemctl --version | sed -n "1s/^systemd \([0-9.]*\).*/\1/p")"
     echo "hyprland=$(pacman -Q hyprland 2>/dev/null | cut -d" " -f2)"
     echo "system_state=$(systemctl is-system-running 2>/dev/null)"
-    echo "failed_units=$(systemctl --failed --no-legend --plain | awk "{print \$1}" | paste -sd, -)"
-    echo "failed_user_units=$(systemctl --user --failed --no-legend --plain | awk "{print \$1}" | paste -sd, -)"
+    # A manager that cannot be asked is not a manager with nothing failed.
+    units=$(systemctl --failed --no-legend --plain 2>/dev/null) && units=$(awk "{print \$1}" <<<"$units" | paste -sd, -) || units="could not ask systemd"
+    user_units=$(systemctl --user --failed --no-legend --plain 2>/dev/null) && user_units=$(awk "{print \$1}" <<<"$user_units" | paste -sd, -) || user_units="could not ask the user manager"
+    echo "failed_units=$units"
+    echo "failed_user_units=$user_units"
     echo "hyprland_running=$(pgrep -c -x Hyprland)"
     echo "migrations_run=$(find ~/.local/state/omarchy/migrations -maxdepth 1 -name "*.sh" | wc -l)"
     echo "migrations_skipped=$(find ~/.local/state/omarchy/migrations/skipped -name "*.sh" 2>/dev/null | wc -l)"
@@ -192,9 +195,14 @@ check "It is on the $channel channel (got: $(a channel))" test "$(a channel)" =
 check "The running kernel is an installed one ($(a kernel))" test "$(a kernel_is_installed)" = yes
 check "systemd reports the system running (got: $(a system_state))" test "$(a system_state)" = running
 failed="$(a failed_units) $(a failed_user_units)"
-check "No failed units (${failed// /}${failed:+ })" test -z "${failed// /}"
+failed=$(xargs <<<"$failed")
+check "No failed units (${failed:-none})" test -z "$failed"
 check "The desktop is up (Hyprland processes: $(a hyprland_running))" test "$(a hyprland_running)" = 1
-check "The wallpaper the user had still exists ($(a wallpaper))" test "$(a wallpaper_exists)" = yes
+# The same picture, whatever format it is kept in now: an update may convert
+# the bundled images, and may not swap the one the user chose for another.
+before_wallpaper=$(b wallpaper) after_wallpaper=$(a wallpaper)
+check "The wallpaper is the one the user had, and exists ($before_wallpaper → $after_wallpaper)" \
+  test "$(a wallpaper_exists)" = yes -a -n "$after_wallpaper" -a "${after_wallpaper%.*}" = "${before_wallpaper%.*}"
 check "The boot menu has one Omarchy group ($(a boot_entry_groups))" test "$(a boot_entry_groups)" = 1
 
 row() { printf '| %s | %s | %s |\n' "$1" "$(b "$2")" "$(a "$2")"; }