scripts/unpack: common directory unpacking is not thread-safe

emby and emby4 both unzip into ${BUILD}/system, which is fun when
both add-ons are being unpacked concurrently.

scripts/unpack

@@ -52,24 +52,40 @@ fi
 if [ -d "$SOURCES/${PKG_NAME}" -o -d "$PKG_DIR/sources" ]; then
   build_msg "CLR_UNPACK" "UNPACK" "${PKG_NAME}" "indent"
 
+  # unpack into a unique location as unpacking into a single ${BUILD} directory is not thread-safe
+  PKG_UNPACK_DIR="${BUILD}/.unpack/${PKG_NAME}"
+  rm -fr "${PKG_UNPACK_DIR}"
+  mkdir -p "${PKG_UNPACK_DIR}"
+
   pkg_call_exists pre_unpack && pkg_call pre_unpack
 
   if pkg_call_exists unpack; then
     pkg_call unpack
   else
     if [ -n "$PKG_URL" ]; then
-      $SCRIPTS/extract "${PKG_NAME}" $BUILD
+      ${SCRIPTS}/extract "${PKG_NAME}" "${PKG_UNPACK_DIR}"
     fi
   fi
 
-  if [ ! -d "$PKG_BUILD" ]; then
-    if [ -n "$PKG_SOURCE_DIR" ]; then
-      mv $BUILD/$PKG_SOURCE_DIR "$PKG_BUILD"
-    elif [ -d "$PKG_BUILD"* ]; then
-      mv "$PKG_BUILD"* "$PKG_BUILD"
+  _pkg_build="$(basename "${PKG_BUILD}")"
+  [ -d "${PKG_UNPACK_DIR}/${_pkg_build}"* ] && mv "${PKG_UNPACK_DIR}/${_pkg_build}"* "${PKG_BUILD}"
+
+  if [ ! -d "${PKG_BUILD}" ]; then
+    if [ -n "${PKG_SOURCE_DIR}" ]; then
+      if [ -d "${PKG_UNPACK_DIR}"/${PKG_SOURCE_DIR} ]; then
+        mv "${PKG_UNPACK_DIR}"/${PKG_SOURCE_DIR} "${PKG_BUILD}"
+      else
+        # fallback
+        mv "${BUILD}"/${PKG_SOURCE_DIR} "${PKG_BUILD}"
+      fi
+    elif [ -d "${PKG_BUILD}"* ]; then
+      mv "${PKG_BUILD}"* "${PKG_BUILD}"
     fi
   fi
 
+  # cleanup
+  rm -fr "${PKG_UNPACK_DIR}"
+
   if [ -d "$PKG_DIR/sources" ]; then
     [ ! -d "$PKG_BUILD" ] && mkdir -p "$PKG_BUILD"
     cp -PRf "$PKG_DIR/sources/"* "$PKG_BUILD"