Make sure the tested dub executable can be found.

Index: test/issue1408-inherit-linker-files/dub.sdl
--- test/issue1408-inherit-linker-files/dub.sdl.orig
+++ test/issue1408-inherit-linker-files/dub.sdl
@@ -10,5 +10,5 @@ subPackage {
 	sourceFiles "dep.d"
 	sourceFiles "lib/liblib.a" platform="posix"
 	sourceFiles "lib/lib.lib" platform="windows"
-	preBuildCommands "$DUB_EXE build --root=\"$PACKAGE_DIR/lib\""
+	preBuildCommands "../../bin/dub build --cache=local --root=\"$PACKAGE_DIR/lib\""
 }