Use installed package to execute buildonly test.

Index: conformance/interfaces/sighold/3-2.sh
--- conformance/interfaces/sighold/3-2.sh.orig
+++ conformance/interfaces/sighold/3-2.sh
@@ -10,4 +10,4 @@
 #   Test various methods of passing invalid signals to sighold().
 # 
 
-conformance/interfaces/sighold/3-core-buildonly.test 2
+${TRUEPREFIX}/libexec/posixtestsuite/conformance/interfaces/sighold/3-core-buildonly.test 2
