fix(ci): nightly tags breaking release changelogs (#126)

After released v0.5.0, I saw that v0.5.0 showed only 1 commit changelog
instead of the real 16 commits since v0.4.21

Nightly builds tag every push to main, so a nightly tag can land on the
exact same commit as a stable release tag
GoReleaser picked that nearby nightly as the `previous release` when
building the changelog

Fixes:

- `.goreleaser.yaml`: added `git.ignore_tags`: ["*-nightly.*"] so
changelog diffing skips nightly tags entirely
- `nightly.yml`: the cleanup step deleted a tag named `nightly`, but
each run tags as `v<version>-nightly.<sha>`, so it never actually
deleted anything and nightly tags piled up unbounded. Now deletes all
previous `v*-nightly.*` tags/releases before creating the new one

.goreleaser.yaml

@@ -2,6 +2,13 @@ version: 2
 
 project_name: iris
 
+git:
+    # nightly tags share the version stream and can land on the same commit
+    # as a stable tag, which would make goreleaser diff the changelog against
+    # a nightly instead of the real previous stable release
+    ignore_tags:
+        - "*-nightly.*"
+
 builds:
     - env:
           - CGO_ENABLED=0