Reported by 宋文武 <iyzsong@gmail.com>.
* guix/build/profiles.scm (build-etc/profile,
ensure-writable-directory): New procedures.
(build-profile): Use them.
* tests/profiles.scm ("etc/profile when etc/ already exists"): New test.
* guix/build/profiles.scm: New file.
* Makefile.am (MODULES): Add it.
* guix/profiles.scm (profile-derivation)[builder]: Call out to
'build-profile'.
Add (guix build profiles) to the #:modules argument.