Use installed package to execute buildonly test.

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