* guix/build/gnu-build-system.scm (configure): Add `inputs' keyword
parameter. Take Bash from there, falling back to /bin/sh. Set
`CONFIG_SHELL' and `SHELL' to that Bash. Run "bash ./configure"
instead of just "./configure".
* distro/packages/bootstrap.scm (%bootstrap-inputs): Add "bash".
* distro/packages/base.scm (gcc-boot0-wrapped): Use "bash" from
%BOOT1-INPUTS instead of /bin/sh.