🚦 set CONTENT_CDN_HOST in env of development server

[stevejpurves]

These changes are to support the fix in https://github.com/executablebooks/myst-theme/pull/346 where url rewriting should honor the HOST variable used to start the mystmd development server.

This PR also changes the existing log messages to accurately report the usage of HOST rather than always citing localhost

[rowanc1]

@stevejpurves how does this work with the --keephost option? I think we might need to change something around here: https://github.com/executablebooks/mystjs/blob/main/packages/myst-cli/src/build/site/start.ts#L85

The host variable is always set to localhost in that block to get around an earlier bug. Does the host variable set both the app and content servers? It looks like this is only targeting the content server?