Use installed package to execute buildonly test.

Index: conformance/interfaces/sigdelset/1-1.sh
--- conformance/interfaces/sigdelset/1-1.sh.orig
+++ conformance/interfaces/sigdelset/1-1.sh
@@ -13,4 +13,4 @@
 
 # Calling 1-core-buildonly.test with a 0 parameter initializes to an
 # empty set.
-conformance/interfaces/sigdelset/1-core-buildonly.test 0
+${TRUEPREFIX}/libexec/posixtestsuite/conformance/interfaces/sigdelset/1-core-buildonly.test 0
