feat: add venv-path input for activate-environment (#746)

Allow customizing the venv location while preserving working-directory
semantics via --directory.

Supersedes: #736

action-types.yml

@@ -9,6 +9,8 @@ inputs:
     type: string
   activate-environment:
     type: boolean
+  venv-path:
+    type: string
   working-directory:
     type: string
   checksum: