add an entry to load general packages from the module system at the very beginning of jobs, e.g. to load git

Including an icon.load_cmd for the cases where the build command includes a hash that can't be used for load. Defaults to the value of icon.build_cmd

Fix build_python_environment.sh adding a new python variable in the configuration file. Also loading the first occurrence of python in case of multiple matches.

try to encapsulate spack.compiler string... problem is that it does get substituted in the icon.spack_cmd