use Geant4-srcs master for version HEAD

Geant4/G4config/CMakeLists.txt

@@ -31,6 +31,10 @@ if(DEFINED ENV{G4_NATIVE_VERSION})
 else()
   set(Geant4_tag ${CMAKE_PROJECT_VERSION})
 endif()
+# Special mapping of version HEAD
+if(Geant4_tag STREQUAL "HEAD")
+  set(Geant4_tag master)
+endif()
 
 include(ExternalProject)