fix(release): include plugin tarball in checksums.txt

GoReleaser only checksums built archives by default, not extra_files.
The plugin tarball was missing from checksums.txt causing install.sh
to fail with "Checksum entry not found" on fresh installs.

Adding extra_files to the checksum section fixes this. [skip release]

Author: automagik-genie

.goreleaser.yaml

@@ -87,6 +87,8 @@ archives:
 
 checksum:
   name_template: checksums.txt
+  extra_files:
+    - glob: _plugin_dist/workit-plugin_*.tar.gz
 
 release:
   extra_files: