rpc/system_metrics.rs: Added rustversion label to garage_build_info metric.

src/util/build.rs

@@ -0,0 +1,8 @@
+use rustc_version::version;
+
+fn main() {
+	// Acquire the version of Rust used to compile, this is added as a label to
+	// the garage_build_info metric.
+	let v = version().unwrap();
+	println!("cargo:rustc-env=RUSTC_VERSION={v}");
+}