Add builder flags for CI and resumed builds

Three opt-in knobs for bin/build, each defaulting to today's behaviour:

OMARCHY_KEEP_BUILD_WORKSPACE=1 keeps build-output/$MIRROR/$ARCH instead
of wiping it, and build/build.sh now folds any packages already there
into omarchy-build.db even when no database exists yet, so packages
built by an earlier job (or a previous, interrupted run) resolve as
dependencies of what builds next.

OMARCHY_SKIP_BUILDER_IMAGE=1 uses the omarchy-pkg-builder image already
present instead of building it, so a workflow can build the image once
with an external BuildKit cache and fan out over package jobs that all
run the same bytes. A missing image is an error, not a silent rebuild.

OMARCHY_DEFER_RUNTIME_DEPS=true builds the omarchy/omarchy-settings pair
with --nodeps, installing only their makedepends and checkdepends
explicitly. The pair depends on each other and on packages a sharded
pipeline builds in other jobs, so they cannot resolve in isolation; the
assembled set is installed in one verified transaction downstream. The
request is refused for anything but exactly that pair, on the host
before Docker starts and again inside the container.

Also fix make_dir_writable: chown -R can succeed on part of the tree
and fail on files a previous container left behind as another uid, and
the old `|| chmod` fallback only ran when chown failed outright. Always
follow with chmod.

helpers/docker-helpers.sh

@@ -61,9 +61,14 @@ get_platform_arg() {
 
 make_dir_writable() {
   local dir="$1"
-  if [ "$(id -u)" -eq 0 ]; then
+  if (( EUID == 0 )); then
     chmod -R 777 "$dir"
   else
-    sudo chown -R $(id -u):$(id -g) "$dir" 2>/dev/null || chmod -R 777 "$dir"
+    # chown can succeed on part of the tree and fail on the rest (files a
+    # previous container left behind as another uid); the old `|| chmod`
+    # fallback only ran when chown failed outright, leaving those files
+    # unwritable. Always follow with chmod so the whole tree is usable.
+    sudo chown -R "$(id -u):$(id -g)" "$dir" 2>/dev/null || true
+    chmod -R 777 "$dir"
   fi
 }