Can't read from `.env` files using `AddEnvFile(string)`

[YuKitsune]

When using the .AddEnvFile(string) method, specifying a .env file, a FileNotFoundException is always returned.

Example:

.ConfigureAppConfiguration((ctx, cfg) =>
  {
      cfg.AddEnvironmentVariables(prefix: "MyApp_");
      cfg.AddEnvFile(".env");
  })

Stacktrace:

System.IO.FileNotFoundException
The configuration file '.env' was not found and is not optional.
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.HandleException(ExceptionDispatchInfo info)
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load(Boolean reload)
   at Microsoft.Extensions.Configuration.FileConfigurationProvider.Load()
   at Microsoft.Extensions.Configuration.ConfigurationRoot..ctor(IList`1 providers)
   at Microsoft.Extensions.Configuration.ConfigurationBuilder.Build()

[YuKitsune]

This is happening because by default, the FileConfigurationSource excludes dot-prefixed files, hidden files, and system files from being read into the configuration.

I've opened an issue on the .NET runtime repo asking why this is the case.
dotnet/runtime#54712

[YuKitsune]

Fixed in #3
The misleading error is being addressed in a PR on the .NET Core runtime PR: dotnet/runtime#54717