Actually rebuild the axes at the beginning of the matrix build

changelog.html

@@ -66,6 +66,9 @@ Upcoming changes</a>
   <li class=bug>
     Single invalid e-mail address shouldn't cause the entire e-mail delivery to fail.
     (<a href="https://github.com/jenkinsci/jenkins/pull/526">pull 526</a>)
+  <li class=bug>
+    Dynamically recomputing matrix axes wasn't working.
+    (<a href="https://github.com/jenkinsci/jenkins/pull/523">pull 523</a>)
   <li class=rfe>
     Make the draggable component more obvious by providing a border.
   <li class=rfe>

core/src/main/java/hudson/matrix/MatrixProject.java

@@ -590,7 +590,7 @@ public class MatrixProject extends AbstractProject<MatrixProject,MatrixBuild> im
         if (context!=null) {
             List<Set<String>> axesList = Lists.newArrayList();
             for (Axis axis : axes)
-                axesList.add(new LinkedHashSet<String>(axis.getValues()));
+                axesList.add(Sets.newLinkedHashSet(axis.rebuild(context)));
 
             activeCombinations = Iterables.transform(Sets.cartesianProduct(axesList), new Function<List<String>, Combination>() {
                 public Combination apply(@Nullable List<String> strings) {