pw_build: Fix Undefined identifier

The `output_name` was removed in http://pwrev.dev/230012. This caused
bootstrape to fail when `pw_rust_static_library` is used.

Change-Id: I12c41d2faa888e451e826e84ecbc592291dd3940
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/232371
Docs-Not-Needed: Min Xu <[email protected]>
Pigweed-Auto-Submit: Min Xu <[email protected]>
Reviewed-by: Jiacheng Lu <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Lint: Lint 🤖 <[email protected]>
Reviewed-by: Erik Gilling <[email protected]>
Reviewed-by: Bjorn Mellem <[email protected]>

pw_build/rust_staticlib.gni

@@ -63,6 +63,6 @@ template("pw_rust_static_library") {
     crate_name = string_replace(crate_name, "-", "_")
     output_dir = "${target_out_dir}/lib"
     add_global_link_deps = true
-    libs = [ "${output_name}" ]
+    libs = [ "${crate_name}" ]
   }
 }