remove dotenv dependency

[syphar]

resolves #1811

We need to add the config to the tests explicitly, which personally I like more than how it was done before.

[syphar]

r? @Nemo157

also @jyn514 (?) to know if this is really not used in prod?

[Nemo157]

In production the systemd unit is configured with:

EnvironmentFile=/home/cratesfyi/.docs-rs-env
WorkingDirectory=/home/ubuntu/docs.rs

The .env loading is relative to working directory (and walks up the directory tree), so checking for that:

> ls /home/ubuntu/docs.rs/.env /home/ubuntu/.env /home/.env /.env
ls: cannot access '/home/ubuntu/docs.rs/.env': No such file or directory
ls: cannot access '/home/ubuntu/.env': No such file or directory
ls: cannot access '/home/.env': No such file or directory
ls: cannot access '/.env': No such file or directory

[syphar]

@Nemo157 thanks for checking! that sounds like we're safe removing this :)

[jyn514]

this seems fine, I guess. I'm probably going to be a bit annoyed by it next time I run docs.rs locally, but I also do that so infrequently I don't think my opinion should matter very much.