ECBUILD-511 Fix regression test source paths

Author: oiffrig

regressions/ECBUILD_511/bundleAB/CMakeLists.txt

@@ -4,7 +4,7 @@ find_package(ecbuild 3.4 REQUIRED)
 
 project(bundleAB VERSION 0.1.0 LANGUAGES C)
 
-ecbuild_bundle(PROJECT projectA SOURCE ../projectA)
-ecbuild_bundle(PROJECT projectB SOURCE ../projectB)
+ecbuild_bundle(PROJECT projectA)
+ecbuild_bundle(PROJECT projectB)
 
 ecbuild_install_project(NAME ${PROJECT_NAME})