ci(flatpak): sync with flathub (#3259)

cmake/targets/common.cmake

@@ -53,10 +53,17 @@ endif()
 
 #WebUI build
 find_program(NPM npm REQUIRED)
+
+if (NPM_OFFLINE)
+    set(NPM_INSTALL_FLAGS "--offline")
+else()
+    set(NPM_INSTALL_FLAGS "")
+endif()
+
 add_custom_target(web-ui ALL
         WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
         COMMENT "Installing NPM Dependencies and Building the Web UI"
-        COMMAND "$<$<BOOL:${WIN32}>:cmd;/C>" "${NPM}" install
+        COMMAND "$<$<BOOL:${WIN32}>:cmd;/C>" "${NPM}" install ${NPM_INSTALL_FLAGS}
         COMMAND "${CMAKE_COMMAND}" -E env "SUNSHINE_BUILD_HOMEBREW=${NPM_BUILD_HOMEBREW}" "SUNSHINE_SOURCE_ASSETS_DIR=${NPM_SOURCE_ASSETS_DIR}" "SUNSHINE_ASSETS_DIR=${NPM_ASSETS_DIR}" "$<$<BOOL:${WIN32}>:cmd;/C>" "${NPM}" run build  # cmake-lint: disable=C0301
         COMMAND_EXPAND_LISTS
         VERBATIM)