Use placeholder to define the integration build repository

Currently we use fixed strings that regularly needs to be updated, but
the contents could actually be computed automatically.

This replace fixed strings with using properties making it needless to
change them all the time.

eclipse-platform-parent/pom.xml

@@ -78,9 +78,9 @@
       'eclipiserun-repo' repository, such as for computing .api-descriptions and
       generating API Tools reports.
     -->
-    <eclipserun-repo>https://download.eclipse.org/eclipse/updates/4.34-I-builds/</eclipserun-repo>
+    <eclipserun-repo>https://download.eclipse.org/eclipse/updates/${releaseVersion}-${buildType}-builds/</eclipserun-repo>
 
-    <comparator.repo>https://download.eclipse.org/eclipse/updates/4.34-I-builds</comparator.repo>
+    <comparator.repo>https://download.eclipse.org/eclipse/updates/${releaseVersion}-${buildType}-builds</comparator.repo>
 
     <!-- only used when Tycho snapshot repo is enabled in <pluginRepositories> further down -->
     <tycho-snapshot-repo.url>https://repo.eclipse.org/content/repositories/tycho-snapshots/</tycho-snapshot-repo.url>