Store checkout location in workspace variable

Update the Eclipse product setup so that `checkout.location` is stored
as a workspace variable. Without this things get mixed up when trying
to run different installations for different branches.

See gh-4110

eclipse/spring-boot-project.setup

@@ -17,6 +17,8 @@
       xsi:type="setup:VariableTask"
       type="FOLDER"
       name="checkout.location"
+      defaultValue=""
+      storageURI="scope://Workspace"
       label="Checkout Location"/>
   <setupTask
       xsi:type="jdt:JRETask"