Ensure the local plugin directory exists

Author: maru-ava

.envrc

@@ -8,4 +8,5 @@ PATH_add build
 export AVALANCHEGO_PATH=$PWD/build/avalanchego
 
 # Configure the local plugin directory for both avalanchego and tmpnet usage
-export AVAGO_PLUGIN_DIR=$PWD/build/plugins
+mkdir -p $PWD/build/plugins                                       # avalanchego will FATAL if the directory does not exist
+export AVAGO_PLUGIN_DIR="${AVAGO_PLUGIN_DIR:-$PWD/build/plugins}" # Use an existing value if set