Allow julia executable to be a symlink

Prior to this patch, the julia backend does not allow the executable to be a symlink. This is an issue if julia is installed using nix (which uses symlinks to install files).

This commit removes this restriction.

By searching the cantor repo for symlink, this restriction only applies to the julia backend. I believe that this was introduced by 1e144559 but the reason for the restriction is not stated.

Edit: I also (previously) created an issue https://bugs.kde.org/show_bug.cgi?id=470381