fix(deploy flash): validate device before building the image

scripts/deploy

@@ -97,6 +97,11 @@ case "$cmd" in
 
   image|flash)
     config="${2:-}"; [ -n "$config" ] || die "usage: ./deploy $cmd <config> [<dev>]"
+    # Validate the device BEFORE building, so a bad `flash` fails fast.
+    if [ "$cmd" = flash ]; then
+      dev="${3:-}"; [ -n "$dev" ] || die "usage: ./deploy flash <config> <dev>  (e.g. /dev/sdX)"
+      [ -b "$dev" ] || die "$dev is not a block device"
+    fi
 
     echo ">> building SD image for .#$config (aarch64 needs qemu binfmt)"
     nix build ".#nixosConfigurations.$config.config.system.build.sdImage" -o result-sd
@@ -104,8 +109,6 @@ case "$cmd" in
     echo ">> image: $img"
     [ "$cmd" = image ] && exit 0
 
-    dev="${3:-}"; [ -n "$dev" ] || die "usage: ./deploy flash <config> <dev>  (e.g. /dev/sdX)"
-    [ -b "$dev" ] || die "$dev is not a block device"
     echo ">> TARGET DEVICE — everything on it will be ERASED:"
     lsblk -o NAME,SIZE,MODEL,TRAN,MOUNTPOINTS "$dev"
     read -rp ">> type 'yes' to write $config to $dev: " ok