Blacklist /usr as bwrap mount point

/usr is not mounted by pressure-vessel-wrap and will print a warning
instead. Follow Steam Runtime behavior by not attempting to mount /usr.

The warning will still appear if either the Steam library or Proton
installation is located under /usr. This is preferable, since it
informs the user that they should move the directory elsewhere.

Also see:
ValveSoftware/steam-runtime#288

src/protontricks/util.py

@@ -139,7 +139,7 @@ def find_runtime_app_root(runtime_app):
 PROTONTRICKS_PROXY_SCRIPT_PATH="{script_path}"
 
 BLACKLISTED_ROOT_DIRS=(
-    /bin /dev /lib /lib64 /proc /run /sys /var
+    /bin /dev /lib /lib64 /proc /run /sys /var /usr
 )
 
 ADDITIONAL_MOUNT_DIRS=(