Updated //tools/rustfmt to dynamically work within rules_rust itself

[UebelAndre]

This should make it less annoying to format .rs files within the rules_rust workspace since previously I'd have to apply a diff to get it to run:

diff --git a/tools/rustfmt/srcs/main.rs b/tools/rustfmt/srcs/main.rs
index f1d95c6..99ef4eb 100644
--- a/tools/rustfmt/srcs/main.rs
+++ b/tools/rustfmt/srcs/main.rs
@@ -76,7 +76,7 @@ fn query_rustfmt_targets(options: &Config) -> Vec<String> {
 fn generate_rustfmt_target_manifests(options: &Config, targets: &[String]) {
     let build_args = vec![
         "build",
-        "--aspects=@rules_rust//rust:defs.bzl%rustfmt_aspect",
+        "--aspects=//rust:defs.bzl%rustfmt_aspect",
         "--output_groups=rustfmt_manifest",
     ];

[illicitonion]

Seems reasonable, given the bazel constraint :)