Fixes #636.

`DefaultEnvars` cleared `Flag.Envs` for `env:"-"` but left the marker in
`Value.Tag.Envs`, which help formatting uses. Clear both representations
and cover the generated help with a regression test.

Tests:
- `go test ./... -run '^TestDisabledDefaultEnvarAutoHelp$' -count=1`
- `go test ./... -run 'EnvarAutoHelp|^TestEnv$|^TestEnvars' -count=1`
- `go test ./...`
- `golangci-lint run --new-from-rev=HEAD`