.env-file support should have a flag to override pre-existing env-variables

I would love to see a setting to read .env-file to override/not-override pre-existing env-variables

e.g:
```
# load environment variables from .env file
set dotenv-filename := ".env"
set dotenv-load := true
set dotenv-enforce:= true # to enforce the values set in .env to override pre-existing env-variables

```

current options:
- dotenv-filename | string | - | Load a .env file with a custom name, if present.
- dotenv-load | boolean | false | Load a .env file, if present.
- dotenv-path | string | - | Load a .env file from a custom path and error if not present. Overrides dotenv-filename.
- dotenv-required | boolean | false | Error if a .env file isn’t found.

see also https://github.com/casey/just/issues/2597

Start by tracing the existing dotenv-filename, dotenv-load, dotenv-path, and dotenv-required settings and the .env loading path. Compare the requested override behavior with the discussion in issue 2597, then add coverage showing how pre-existing environment variables are handled and verify both override and non-override modes.

Written by the indexing model from the issue text.