Hi Guix,

I slightly botched the last numpy update: setting $SHELL works, but 
then we have to set $SHELL for every package which uses numpy's 
distutils.  I don't know how many packages there are, but it includes 
scipy and python-hdf (and therefore those builds are currently broken 
on master...).

The attached patch should fix the issue for all packages, by setting 
the default shell to the “/bin/sh” of the build environment's bash 
package (i.e. bash-minimal).

Thomas