omarchy-dev-pkg-test: install with pacman -U --overwrite=*; add plymouth to transition overwrites

Two related fixes for installing package builds onto a system that has
the legacy script-installed Omarchy files already in place:

bin/omarchy-dev-pkg-test: split makepkg -s (build) from sudo pacman -U
(install) so we can pass --overwrite='*' to pacman. makepkg -i forwards
a fixed flag set to pacman and doesn't expose --overwrite, so dev pkg
tests on an existing script-install fail on every conflicting file
(plymouth themes, /etc drop-ins, sudoers). --overwrite='*' is correct
for the dev use case: the build IS the new authoritative state.

bin/omarchy-update-system-pkgs: add /usr/share/plymouth/themes/omarchy/*
to the transition --overwrite list. The old install/login/plymouth.sh
cp -r'd the theme files into that directory, so existing installs
conflict on the omarchy-settings package's plymouth payload.

bin/omarchy-update-system-pkgs

@@ -30,4 +30,5 @@ sudo pacman -Syyu --noconfirm \
   --overwrite '/etc/systemd/system/user@.service.d/10-faster-shutdown.conf' \
   --overwrite '/etc/systemd/system/docker.service.d/no-block-boot.conf' \
   --overwrite '/etc/systemd/system/plocate-updatedb.service.d/ac-only.conf' \
-  --overwrite '/usr/lib/systemd/system-sleep/unmount-fuse'
+  --overwrite '/usr/lib/systemd/system-sleep/unmount-fuse' \
+  --overwrite '/usr/share/plymouth/themes/omarchy/*'