Since this hasn't received any replies I'll give my solution, although I
apologize in advance if this isn't the "best" way to do it.
When I want to modify a package that I install via "guix package -i" (as opposed
to using the GuixSD system configuration) I just create a new file for each such
package that inherits the default package and then override any options I need.
For example, in the case of curl you could make a file with the contents:
--------------------
(use-modules ((guix licences) #:prefix license:)
(gnu packages)
(gnu packages curl)
(guix packages)
(guix utils)))
(package
(inherit curl)
(substitute-keyword-arguments (package-arguments curl)
((#:tests? #f #f)
#f)))
--------------------
which will just inherit curl and only modify the flag "tests?" to disable tests.
Everything else will be kept the same. Then if you save that to a file called
my-curl.scm you could install it to your guix profile with
"guix package -f my-curl.scm"
