completion script should honor env var for desired Python ver

[neogeographica]

The shortcut scripts can pay attention to the CHAINTOOL_SHORTCUT_PYTHON var to pick the right version of Python for launching chaintool. The completion script should probably do the same (when using it to do dump_placeholders).

(and maybe then just rename the var to CHAINTOOL_PYTHON)

[neogeographica]

done