build: Use find_program to find NPM so it can be overridden

This is useful for Gentoo, which needs to be able to do entirely offline
builds, because it can override this with `true` while shipping
pre-compiled assets. Gentoo has tried to ship cached NPM modules
instead, but it turns out these are very sensitive to the NPM version.

cmake/targets/common.cmake

@@ -16,11 +16,12 @@ else()
 endif()
 
 #WebUI build
+find_program(NPM npm REQUIRED)
 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 "${CMAKE_COMMAND}" -E env "SUNSHINE_BUILD_HOMEBREW=${NPM_BUILD_HOMEBREW}" "SUNSHINE_SOURCE_ASSETS_DIR=${NPM_SOURCE_ASSETS_DIR}" "SUNSHINE_ASSETS_DIR=${NPM_ASSETS_DIR}" "$<$<BOOL:${WIN32}>:cmd;/C>" npm run build  # cmake-lint: disable=C0301
+        COMMAND "$<$<BOOL:${WIN32}>:cmd;/C>" "${NPM}" install
+        COMMAND "${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)