Fix the 'Bundle Pool' option for the Eclipse Installer

Before this commit, if someone ticked the 'Bundle Pool' option in the
Eclipse Installer while installing Epsilon, the target platform would
stop resolving and show 0 plugins.

Comparing the Oomph file used to install Eclipse products and our own
configuration, I found the first one to include 'Installation' and 'P2
Director' elements at the root, in addition to the specific configs.
Copying these elements to our Oomph file fixed the issue.

releng/org.eclipse.epsilon.releng/epsilonUse.setup

@@ -139,6 +139,16 @@
     </setupTask>
     <description>Creates the workspace and points Eclipse to it, with default preferences pre-configured.</description>
   </setupTask>
+  <setupTask
+      xsi:type="setup:InstallationTask"
+      id="installation"/>
+  <setupTask
+      xsi:type="setup.p2:P2Task">
+    <requirement
+        name="org.eclipse.oomph.setup.feature.group"/>
+    <repository
+        url="${oomph.update.url}"/>
+  </setupTask>
   <version name="2.6"
       requiredJavaVersion="17">
     <setupTask