Some packages fail to build due to missing shell package

When running autobuilder on gwyn using zsh as root's login shell, slibtool failed to build during stage 1 with the following error:

slibtool : configure : config_host()  .............................  make: /bin/zsh: No such file or directory
make: *** [Makefile:286: host.tag] Error 127
configure was able to generate a Makefile for the selected host,
however the host-targeting compiler was found to be missing
at least one of the required headers or features.
>>> ERROR: slibtool: build failed

This is because SHELL is still defined to /bin/zsh in the sub shell.