fix(install): pin HOME=/root so root nix never strays into /home/nomarchy

`exec sudo --preserve-env` (needed to carry the NOMARCHY_* vars) also
drags the live session user's HOME=/home/nomarchy into the root install
process. Root-run `nix build` then writes an eval cache + .nix-defexpr
there, and the in-chroot one lands on the TARGET as a stray, orphaned
/home/nomarchy (no such user on the installed system). Pin root's own
HOME after the sudo re-exec, and again inside the nixos-enter chroot
script. The user activation already sets HOME=/home/$USERNAME and is
unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pkgs/nomarchy-install/nomarchy-install.sh

@@ -54,6 +54,15 @@ if [[ $EUID -ne 0 ]]; then
     exec sudo --preserve-env "$0" "$@"
 fi
 
+# `sudo --preserve-env` (needed to carry the NOMARCHY_* vars) also drags in
+# the live session user's HOME=/home/nomarchy. Root-run `nix` calls below
+# would then scribble an eval cache + .nix-defexpr into /home/nomarchy —
+# and the in-chroot one lands on the TARGET disk as a stray, orphaned
+# /home/nomarchy (no such user on the installed system). Pin root's own
+# HOME so every root nix invocation stays in /root; the user activation
+# sets HOME=/home/$USERNAME explicitly and is unaffected.
+export HOME=/root
+
 header "Nomarchy installer" "NixOS, themed and ready to go."
 
 [[ -d /sys/firmware/efi ]] \
@@ -628,6 +637,8 @@ cat > /mnt/root/nomarchy-hm-activate.sh <<EOF
 set -ex
 exec > /var/log/nomarchy-hm-preactivate.log 2>&1
 export PATH=/run/current-system/sw/bin:\$PATH
+# Keep root's nix state in /root, not a stray /home/nomarchy on the target.
+export HOME=/root
 # Normally pre-built in the live env and copied over; the in-chroot
 # build (default substituters — the live-side build already proved the
 # no-network case) is a last-resort fallback.