fix TP PR jobs so they run p2diff against last available published TP

Realized today that the TP PR job is failing because it's trying to diff against a TP URL that doesn't (yet) exist.

So, pass in these params:

-DtargetRepositoryURL=\
http://download.jboss.org/jbosstools/targetplatforms/jbosstoolstarget/${TARGET_PLATFORM_VERSION_MAX}/REPO/ \
-Dp2diff.skip=false

If this doesn't work we can just skip the p2diff check entirely.