gnum4: Fix impurity in shell handling

If gnum4 is built outside of a chroot, it will decide to use
/run/current-system/sw/bin/sh as the shell for "syscmd". (It gets this
path via "getconf PATH". Maybe our Glibc shouldn't return that path,
at least not during Nix builds...) If such a build of gnum4 is
subsequently used *inside* a chroot, it won't work because
/run/current-system doesn't exist. So specify an explicit path to the
shell.