fix: tag sorting

When the HEAD commit has multiple tags, they are sorted in order to select the latest so that it can be released.

However, the existing sort would not work if there were multiple commits across a Wednesday and a Thursday.

For example:

```
git tag --points-at HEAD --format='%(creatordate)%09%(refname)'
Wed Jul 28 07:13:19 2021 +0000  refs/tags/v0.0.183
Thu Jul 29 13:31:07 2021 +0000  refs/tags/v0.0.184
Thu Jul 29 13:38:42 2021 +0000  refs/tags/v0.0.185
Thu Jul 29 13:57:44 2021 +0000  refs/tags/v0.0.186
```

When using the existing sort the `creatordate` field was targeted and reversed. Alphabetically Thursday comes before Wednesday, so that is reversed and the Wednesday release always comes first:
```
git tag --points-at HEAD --sort=-version:creatordate --format='%(creatordate)%09%(refname)'
Wed Jul 28 07:13:19 2021 +0000  refs/tags/v0.0.183
Thu Jul 29 13:57:44 2021 +0000  refs/tags/v0.0.186
Thu Jul 29 13:38:42 2021 +0000  refs/tags/v0.0.185
Thu Jul 29 13:31:07 2021 +0000  refs/tags/v0.0.184
```

This would make goreleaser attempt to release that existing tag again, and fail.

If we instead sort by reversed `refname` we get the tags ordered by their numeric value, which ignore the day of the week of release:
```
git tag --points-at HEAD --sort=-version:refname --format='%(creatordate)%09%(refname)'
Thu Jul 29 13:57:44 2021 +0000  refs/tags/v0.0.186
Thu Jul 29 13:38:42 2021 +0000  refs/tags/v0.0.185
Thu Jul 29 13:31:07 2021 +0000  refs/tags/v0.0.184
Wed Jul 28 07:13:19 2021 +0000  refs/tags/v0.0.183
```

Allowing the latest version, 0.0.186 in this case, to be targeted for release.

internal/pipe/git/git.go

@@ -177,7 +177,7 @@ func getTag() (string, error) {
 			return os.Getenv("GORELEASER_CURRENT_TAG"), nil
 		},
 		func() (string, error) {
-			return git.Clean(git.Run("tag", "--points-at", "HEAD", "--sort", "-version:creatordate"))
+			return git.Clean(git.Run("tag", "--points-at", "HEAD", "--sort", "-version:refname"))
 		},
 		func() (string, error) {
 			return git.Clean(git.Run("describe", "--tags", "--abbrev=0"))