Forward OMARCHY_RC_PINS into the build container

package_builds_for_mirror gates pinned packages' rc builds on
OMARCHY_RC_PINS, but that check runs inside the container via
build.sh, and docker run only forwarded ARCH/MIRROR/PACKAGES —
so the rc trigger's pinned builds were always skipped as 'not
configured for direct rc builds'.

bin/build

@@ -143,6 +143,7 @@ DOCKER_ARGS=(
   -e ARCH="$ARCH"
   -e MIRROR="$MIRROR"
   -e PACKAGES="$PACKAGES"
+  -e OMARCHY_RC_PINS="${OMARCHY_RC_PINS:-}"
   -v "$BUILD_ROOT/build-output:/build-output"
   -v "$REPO_ROOT:/pkgs.omarchy.org"
   -v "$BUILD_DIR:/build:ro"