integration test isolation (knative#2894)

- Default builder and pusher set to embedded Host Builder/Pusher(oci)
- Most tests clear environment
- Environment defaults can be controlled via environment variables
- Tests which require back-compat `git` binary actively check and skip
  when running with a cleared environment (both integration and unit).
- Bugfixes for when run in tandem with E2E tests
- Ignores go-created directories in the default home path (testdata)

Author: lkingland

.gitignore

@@ -10,11 +10,14 @@
 /hack/bin
 /.artifacts
 
-/e2e/testdata/default_home/go
-/e2e/testdata/default_home/.cache
-
+/pkg/functions/testdata/migrations/*/.gitignore
+/pkg/functions/testdata/default_home/go
+/pkg/functions/testdata/default_home/.cache
 /pkg/functions/testdata/migrations/*/.gitignore
 
+# Go
+/templates/go/cloudevents/go.sum
+
 # JS
 node_modules
 /templates/typescript/cloudevents/build
@@ -25,10 +28,12 @@ __pycache__
 /templates/python/cloudevents/.venv
 /templates/python/http/.venv
 
-# VSCode
+# E2E Tests
+/e2e/testdata/default_home/go
+/e2e/testdata/default_home/.cache
+ 
+# Editors
 .vscode
-
-# IntelliJ
 .idea
 
 # Operating system temporary files