playbooks: Split out the post-configuration steps into a separate file

A subsequent commit will introduce builds performed with the
-Dmigration_path_for_coreos_toolbox option to the CI.  It will be good
to avoid duplicating the build and installation steps for builds with
and without the -Dmigration_path_for_coreos_toolbox option.

https://github.com/containers/toolbox/pull/1095

playbooks/build.yaml

@@ -0,0 +1,12 @@
+- name: Build Toolbox
+  command: ninja -C builddir
+  args:
+    chdir: '{{ zuul.project.src_dir }}'
+    creates: builddir/src/toolbox
+
+- name: Install Toolbox
+  become: yes
+  command: ninja -C builddir install
+  args:
+    chdir: '{{ zuul.project.src_dir }}'
+    creates: /usr/local/bin/toolbox

playbooks/setup-env.yaml

@@ -8,15 +8,4 @@
       args:
         chdir: '{{ zuul.project.src_dir }}'
 
-    - name: Build Toolbox
+    - include_tasks: build.yaml
-      command: ninja -C builddir
-      args:
-        chdir: '{{ zuul.project.src_dir }}'
-        creates: builddir/src/toolbox
-
-    - name: Install Toolbox
-      become: yes
-      command: ninja -C builddir install
-      args:
-        chdir: '{{ zuul.project.src_dir }}'
-        creates: /usr/local/bin/toolbox