Updates don't seem to work

So, I installed `github.com/ncruces/zenity` a couple of days ago like so:
```
$ brew install FiloSottile/gomod/brew-gomod
$ brew gomod github.com/ncruces/zenity/cmd/zenity
```

And it worked fine. I released a new version today, so it was time to try and upgrade, this is what I get:
```
$ brew gomod github.com/ncruces/zenity/cmd/zenity
Unlinking /Users/cruces/homebrew/Cellar/gomod-zenity/2021-05-06... 1 symlinks removed.
go: creating new go.mod: module gomod-zenity/2021-05-07
go: downloading github.com/ncruces/zenity v0.7.2
go get: added github.com/ncruces/zenity v0.7.2
Linking /Users/cruces/homebrew/Cellar/gomod-zenity/2021-05-06... 1 symlinks created.
```

Note how `brew link` links the old version again.
A clean uninstall then install works, but any idea how this might be fixed?

Start by reproducing the upgrade with the shown `brew gomod github.com/ncruces/zenity/cmd/zenity` commands and compare the generated `go.mod` module version with the Homebrew cellar and symlink paths in the output. Trace the `brew gomod` entry point through its unlink, install, and link steps; done means upgrading links the newly installed version instead of restoring the old one.

Written by the indexing model from the issue text.