build: add 'cap_lints' flag for rust crates

Using a specialized flag rather than the generic 'args' option makes
build_extra/rust/BUILD.gn shorter and more readable.

build_extra/rust/rust.gni

@@ -56,6 +56,7 @@ template("_rust_crate") {
 
   forward_variables_from(invoker,
                          [
+                           "cap_lints",
                            "cfg",
                            "crate_name",
                            "crate_type",
@@ -280,6 +281,12 @@ template("_rust_crate") {
     if (rust_treat_warnings_as_errors) {
       args += [ "-Dwarnings" ]
     }
+    if (defined(cap_lints)) {
+      args += [
+        "--cap-lints",
+        cap_lints,
+      ]
+    }
     if (defined(invoker.args)) {
       args += invoker.args
     }