env: dump to dotenv

Instead of https://github.com/octomation/maintainer/blob/dbfaab2b34ba9d2363d3e665b20c8bb5783581b1/internal/config/tool_test.go#L44-L46

```go
vars.Dump(writer, [grouper])
```

`grouper` allows to sort and split them by prefix, e.g.

```dotenv
GITHUB_VAR1=...
GITHUB_VAR2=...

BITBUCKET_VAR1=...
...
```