passing args to post build hook

[shubhbapna]

Downstream we have a post build hook that is used to upload the built wheels to our index. This hook relies on env variables to get the required arguments. We should add an option in fromager to pass these variables instead of relying on env variables only. An example design could be:

fromager build-sequence --post-build-hook-arg "KEY1=VAL1" --post-build-hook-arg "KEY2=VAL2"