Use installed package to execute buildonly test.

Index: conformance/interfaces/sigismember/5-2.sh
--- conformance/interfaces/sigismember/5-2.sh.orig
+++ conformance/interfaces/sigismember/5-2.sh
@@ -9,4 +9,4 @@
 #   Test various methods of checking for invalid signals using sigismember().
 # 
 
-conformance/interfaces/sigismember/5-core-buildonly.test 2
+${TRUEPREFIX}/libexec/posixtestsuite/conformance/interfaces/sigismember/5-core-buildonly.test 2
