Error message for missing tor uses correct env var names

[stevenengler]

See ticket #33041.

When you attempt to run Chutney using a command like:

./chutney configure networks/basic

it will print the following error message if it cannot find tor or tor-gencert:

Cannot find the tor-gencert binary at 'tor-gencert' for the command
line 'tor-gencert --create-identity-key --passphrase-fd 0 <snip>'.
Set the TOR_DIR environment variable to the directory containing
tor-gencert.

Chutney (the Python code) does not actually use this TOR_DIR environment variable (it's only used by tools/test-network.sh and related scripts). Instead the error message should say that the user should set the CHUTNEY_TOR and CHUTNEY_TOR_GENCERT environment variables.