radare2: fix update script to handle invocation from elsewhere

It looks like `with Path:` is supposed to be a context-manager
for changing directory but it doesn't seem to work and the only
way I've found to make that work is if we made our own and annotated it.

Instead just use nix search path to tell the expression where
to find the file it looks for.

Also re-use SCRIPT_DIR elsewhere instead of re-"computing" it.