Fixes <http://bugs.gnu.org/25775>.
Reported by Sirgazil, Ricardo Wurmus, Andy Wingo, and others.
* guix/download.scm (in-band-download): Always use GUILE. This reverts
9f05908fb1.
* gnu/packages/bash.scm (url-fetch/reset-patch-level): Remove #:guile.
Pass %BOOTSTRAP-GUILE as #:guile to 'url-fetch'. Remove #:guile-for-build
argument in 'gexp->derivation' call.