Add option to use login shell to build executables

[lucamar]

Currently ReFrame invokes /bin/bash without -l in the build scripts.
It could be useful to add an option to select a login shell as well if needed, i.e. using the option -l of the bash command.

[vkarak]

@lucamar Actually this is an inconsistency with what we generate for jobs. So we can always emit -l for the build scripts as well. Do you think this is going to hurt somewhere?

[lucamar]

@vkarak That's a good idea, I don't think that's going to hurt and it will make the builds easier.