Hi,

> 2. Clone the Guix repo myself, change the file, and somehow replace the upstream channel with my local repo path

$ git clone -b master --depth 1 https://git.savannah.gnu.org/git/guix.git guix-foobar
$ cd guix-foobar
$ guix environment --pure guix --ad-hoc git guile-readline guile-json nano
(env)$ ./bootstrap
(env)$ ./configure --localstatedir=/var
(env)$ make -j5
# Make sure it succeeds.
# Lately it was broken for unrelated reasons (po files)--you don't want to
# confuse those errors with errors your change could have caused.
(env)$ nano gnu/packages/baz.scm
# Edit your package or whatever it is
(env)$ make -j5
(env)$ exit
$ ./pre-inst-env guix build -K blah
# On build failure, examine /tmp/guix-build-blah* directory
# If you want, install the new package into your profile:
$ ./pre-inst-env guix package -i blah