speedup: move when block to outside include

Instead of including a file, then running a conditional in the block
which gets applied inside the task, which then results in every task
running, but getting skipped (especially in scenarios where the build
host is the same as the current one), we simply include if we have to.

This saves around 20 seconds in every iteration of python_venv_build
on metal.

Change-Id: I8afb2970ee696f0fe9a9e250f19f1db465d63511

tasks/main.yml

@@ -19,6 +19,7 @@
 
 - import_tasks: "python_venv_wheel_build.yml"
   run_once: yes
+  when: venv_wheel_build_enable | bool
   tags:
     - build
 

tasks/python_venv_wheel_build.yml

@@ -16,8 +16,6 @@
 - name: Build the wheels on the build host
   delegate_to: "{{ venv_build_host }}"
   become: "{{ venv_build_host == 'localhost' }}"
-  when:
-    - venv_wheel_build_enable | bool
   block:
     - name: Install distro packages for wheel build
       package: