Find real compiler path during compile time

This is a compile-time fix to find the correct nest-config --compiler. This is required for correct function of pynestml on the EBRAINS Jupyter since compile time view of the GCC is not available at run-time.

Partially fixes #51 (closed)

packages/nest/nest-simulator-3.6-p1-CxxRealPath.patch

+diff --git a/bin/nest-config.in b/bin/nest-config.in
+index a54ae0352..43bff5286 100755
+--- a/bin/nest-config.in
++++ b/bin/nest-config.in
+@@ -71,7 +71,7 @@
+         echo "-L$prefix/@CMAKE_INSTALL_LIBDIR@/nest @MODULE_LINK_LIBS@"
+         ;;
+     --compiler)
+-        echo "@CMAKE_CXX_COMPILER@"
++        echo "@SPACK_CXX_COMPILER@"
+         ;;
+     --compiler-name)
+         echo "@CMAKE_CXX_COMPILER_ID@"