feat: change experimental support for dotenv files

[ldez]

For lego run, I added a flag --env-file=.env.
The .env is not loaded by default.

For lego, I removed the support of implicit dotenv files. The environment variables are loaded from the dotenv file defined by envFile in the configuration file for the DNS provider.

For lego, I removed the support of .env file, and changed the pattern to .env.lego.XXX.
I added envFilesDirectory inside the configuration file to change the directory that contain the dotenv files.

Related to #2974

[ldez]

I'm not happy with envFilesDirectory option + dotenv patterns.

I need to think more about the topic.

[ldez]

My initial goal was to have something "transparent" by using a convention.
But the convention seems more complex than an explicit path.

Also the envFilesDirectory option has the problem of "path relativity" I explained here.

So, in this context, I think adding an envFile option on the DNS challenge configuration is a better solution: no conventional pattern, and the same problem of "path relativity" than envFilesDirectory.

Less magic and with some know limitations, but easier to use.