COMP: Build against ITK main to pick up CI fixes

Update itk-git-tag from v6.0a02 to main. The pinned v6.0a02 predates
ccache and dashboard script fixes merged into ITK main.

Author: hjmjohnson

.github/workflows/build-test-publish.yml

@@ -3,7 +3,7 @@ name: Build, test, publish
 on: [push,pull_request]
 
 env:
-  itk-git-tag: "v6.0a02"
+  itk-git-tag: "main"
 
 jobs:
   build-test-cxx: