meson: Set build.rust_std

This is needed because meson currently makes rust_std part of the build
environment and therefore per-arch.  This is a bit nonsense but it's not
too hard to work around.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

meson.build

@@ -24,7 +24,14 @@ project(
   version : files('VERSION'),
   license : 'MIT',
   meson_version : '>= 0.60',
-  default_options : ['buildtype=debugoptimized', 'b_ndebug=if-release', 'c_std=c11', 'cpp_std=c++17', 'rust_std=2021']
+  default_options : [
+    'buildtype=debugoptimized',
+    'b_ndebug=if-release',
+    'c_std=c11',
+    'cpp_std=c++17',
+    'rust_std=2021',
+    'build.rust_std=2021',
+  ],
 )
 
 cc = meson.get_compiler('c')