fix: collaborator can edit wiki with write access (#8234)

fixes: #8119, replaces #8135

Bug likely introduced in 5eeccecafc

### Tests

- I added test coverage for Go changes in the `tests/integration` directory if it involves interactions with a live Forgejo server.

### Documentation

- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8234
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

routers/web/repo/setting/setting.go

@@ -153,11 +153,9 @@ func UnitsPost(ctx *context.Context) {
 		})
 		deleteUnitTypes = append(deleteUnitTypes, unit_model.TypeWiki)
 	} else if form.EnableWiki && !form.EnableExternalWiki && !unit_model.TypeWiki.UnitGlobalDisabled() {
-		var wikiPermissions repo_model.UnitAccessMode
+		wikiPermissions := repo_model.UnitAccessModeUnset
 		if form.GloballyWriteableWiki {
 			wikiPermissions = repo_model.UnitAccessModeWrite
-		} else {
-			wikiPermissions = repo_model.UnitAccessModeRead
 		}
 		units = append(units, repo_model.RepoUnit{
 			RepoID:             repo.ID,