litrpc+proto: fix ownership in docker container

This was discovered while debugging the reproducible build. We used the
wrong group ID. And the environment variables aren't super portable so
we use the correct command to get the numeric user and group IDs.

litrpc/gen_protos_docker.sh

@@ -17,7 +17,6 @@ docker build -t lit-protobuf-builder \
 echo "Compiling and formatting *.proto files..."
 docker run \
   --rm \
-  --user $UID:$UID \
-  -e UID=$UID \
+  --user $(id -u):$(id -g) \
   -v "$DIR/../:/build" \
   lit-protobuf-builder

proto/gen_protos_docker.sh

@@ -11,7 +11,6 @@ docker build -q -t lit-protobuf-builder .
 echo "Compiling and formatting *.proto files..."
 docker run \
   --rm \
-  --user $UID:$UID \
-  -e UID=$UID \
+  --user $(id -u):$(id -g) \
   -v "$DIR/../:/build" \
   lit-protobuf-builder