This would allow to, when multiple tokens are set in the environment,
force which one you want to use.
The need for this comes from the fact gitea sets both `GITHUB_TOKEN` and
`GITEA_TOKEN`, and doesn't allow to easily disable either.
With this, users can add a `GORELEASER_FORCE_TOKEN=gitea` to force the
gitea client to be used.
I'm not sure what's the best name for this env yet, happy to hear
suggestions.
Also improved the `env_test.go` file a bit, as it was kinda messy with
env vars...
refs https://github.com/orgs/goreleaser/discussions/3900
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Signed-off-by: Carlos A Becker <caarlos0@users.noreply.github.com>
