src/lparse: allow btl_folder to be read from environment

We can now use the environment variable `TYPER_BUILTINS` to specify
the folder in which to find `builtins.typer` and `pervasive.typer`.
Having this setting as a ref cell wasn't really helping, because it's
used at the top level to bind `default_ectx`; once you hit
`parse_args()` in `main`, it's too late to change it. So specifying as
a command-line argument will require more changes.