Use arch, not uname, for Linux architecture detection

This actually works in our docker image.

Author: illicitonion

src/python/pants/engine/native_engine_version

@@ -1 +1 @@
-aeffd63979f2584ff3790fe16e343c97efdb1ade
+ba977a99820e0e68944988a098eceab523170195

src/rust/engine/process_executor/bazel_protos/generate-grpc.sh

@@ -13,7 +13,7 @@ case "$(uname)" in
     ;;
   "Linux")
     os="linux"
-    os_version="$(uname -p)"
+    os_version="$(arch)"
     ;;
   *)
     echo >&2 "Unknown platform when fetching protoc"