Use installed package to execute buildonly test.

Index: conformance/interfaces/sigaddset/1-2.sh
--- conformance/interfaces/sigaddset/1-2.sh.orig
+++ conformance/interfaces/sigaddset/1-2.sh
@@ -15,4 +15,4 @@
 
 # Calling 1-core-buildonly.test with a 1 parameter initializes to an
 # full set.
-conformance/interfaces/sigaddset/1-core-buildonly.test 1
+${TRUEPREFIX}/libexec/posixtestsuite/conformance/interfaces/sigaddset/1-core-buildonly.test 1
