Do not read DISPATCH_API_KEY

[chriso]

We no longer accept an API key via DISPATCH_API_KEY, since it overrides the local configuration and makes it too easy to use the wrong key.

The API key is read from the local YAML configuration (populated by dispatch login). An override can still be provided to all commands via -k,--api-key. If the user really wants to use the value of the DISPATCH_API_KEY environment variable, they can explicitly opt-in by invoking dispatch with --api-key $DISPATCH_API_KEY.

Note that dispatch run sets/overrides the value of DISPATCH_API_KEY before spawning the local application. There's no need to strip the value from the environment, though we could perhaps print a warning if it's set to something other than the value found in local configuration or the one passed to -k,--api-key ?

cc #8.

[chriso]

Closing this because we have a use case in mind.