www/*chromium: fix build when rust is not installed in time

VAR!= gets expanded on the first make invocation and because
rust is a build dependency, it might not be installed at that
time, so getting the version of rustc was erratic

use gn to get the version instead

PR: 290619
(cherry picked from commit ee86324cade722c6cb8013545e6b8a8c6b05cf82)