build: Fix handling of missing BLAS.

find_package for BLAS set BLAS_LIBRARY to a literal FALSE.  As used further in the ROOT cmake files
this lead to a link line for a couple of sofie test containing .  However the value is not
cached and thus after a second configuration the problem went away.

cmake/modules/SearchInstalledSoftware.cmake

@@ -1643,6 +1643,9 @@ if(tmva)
       else()
         set(tmva-cpu OFF CACHE BOOL "Disabled because BLAS was not found (${tmva-cpu_description})" FORCE)
       endif()
+      set(BLAS_LIBRARIES) # find package set this variable to a literal FALSE when not found.
+    else()
+      set(BLAS_LIBRARIES BLAS::BLAS)
     endif()
   else()
     set(tmva-cpu OFF CACHE BOOL "Disabled because 'imt' is disabled (${tmva-cpu_description})" FORCE)