rust: build paste dependency with the correct edition

Although a crate may happen to be compatible with multiple editions, building with the wrong
edition can - generally speaking - lead to subtle bugs.

There are no known failures in this case, but better to match the official Cargo.toml anyway.

Fixes: dde95fc039 ("meson,ci: Add the paste crate")
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38726>

subprojects/packagefiles/paste-1-rs/meson.build

@@ -9,7 +9,8 @@ rust = import('rust')
 
 lib = rust.proc_macro(
   'paste',
-  files('src/lib.rs')
+  files('src/lib.rs'),
+  override_options : ['rust_std=2018', 'build.rust_std=2018']
 )
 
 dep_paste = declare_dependency(