installer-iso: force script stage 1 — systemd initrd has no findiso

terra booted the staged installer this time but dropped to an emergency
shell: stage 1 mounted /sysroot, then timed out on /sysroot/nix/.ro-store
waiting for /dev/disk/by-label/nixos-minimal-26.05-x86_64.

findiso= is handled only in the scripted stage-1-init.sh, which loop-mounts
the file the param points at and symlinks it to /dev/root. The systemd initrd
— the default since 26.05 — has no findiso handling at all: iso-image.nix
mounts /iso directly from /dev/disk/by-label/<volumeID>, a label that only
exists when the ISO is the physical boot medium. Booted as kernel + initrd
off the ESP with the iso as a plain file on another partition, that label
never appears, so the store squashfs never mounts.

The entire `install <config> localhost` path is built on findiso, so pin the
installer to script stage 1. Verified: /iso device flips to /dev/root,
root=LABEL=... is added to the params, the rebuilt initrd's /init is
stage-1-init.sh and carries the findiso logic.

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

flake.nix

@@ -208,6 +208,19 @@
               networking.hostName = "homelab-installer";
               environment.systemPackages = [ pkgs.git ];
 
+              # findiso= is a SCRIPT-stage-1 feature (stage-1-init.sh) only. The
+              # systemd initrd — the default since 26.05 — has no findiso path
+              # at all: it mounts /iso straight from
+              # /dev/disk/by-label/<volumeID> (iso-image.nix), which only exists
+              # when the ISO is the physical boot medium. Booted as a kernel +
+              # initrd off the ESP with the iso as a plain file elsewhere, that
+              # label never appears and stage 1 times out into an emergency
+              # shell (mounts /sysroot fine, then fails /sysroot/nix/.ro-store).
+              # Script stage 1 instead loop-mounts the file findiso= points at
+              # and symlinks it to /dev/root — which is the whole mechanism this
+              # install path relies on. So force it off here.
+              boot.initrd.systemd.enable = false;
+
               # installation-cd-minimal leaves experimental-features unset, so
               # the ISO's nix.conf has no `nix-command`/`flakes` at all (unlike
               # the nixos-images kexec installer, which sets