Additional channels pulled, but cannot install/update software

["Simen Endsjø" <simendsjo@gmail.com>]

I'm having a problem where `guix pull` pulls all channels defined in
~/.config/guix/channels.scm, but somehow only guix is available thereafter.
Any idea what might have happened? I have software installed through these
channels which now cannot be updates as they don't exist in the main guix
channel.

    $ cat ~/.config/guix/channels.scm
    (cons* (channel
            (name 'nonguix)
            (url "https://gitlab.com/nonguix/nonguix")
            (introduction
            (make-channel-introduction
            "897c1a470da759236cc11798f4e0a5f7d4d59fbc"
            (openpgp-fingerprint
            "2A39 3FFF 68F4 EF7A 3D29  12AF 6F51 20A0 22FB B2D5"))))
        (channel
            (name 'flat)
            (url "https://github.com/flatwhatson/guix-channel.git")
            (introduction
            (make-channel-introduction
            "33f86a4b48205c0dc19d7c036c85393f0766f806"
            (openpgp-fingerprint
            "736A C00E 1254 378B A982  7AF6 9DBE 8265 81B6 4490"))))
        %default-channels)


    $ guix describe --format=channels
    (list (channel
            (name 'guix)
            (url "https://git.savannah.gnu.org/git/guix.git")
            (branch "master")
            (commit
            "a43c3b1818db4860ec81bf867b1e53b418a0acec")
            (introduction
            (make-channel-introduction
                "9edb3f66fd807b096b48283debdcddccfea34bad"
                (openpgp-fingerprint
                "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA")))))


    $ guix pull --list-generations
    # ...
    Generation 28        Jun 24 2022 21:00:31 (current)
    nonguix 1a122e0
        repository URL: https://gitlab.com/nonguix/nonguix
        branch: master
        commit: 1a122e06fe046caebf39395edc797515861acd3b
    flat 657da22
        repository URL: https://github.com/flatwhatson/guix-channel.git
        branch: master
        commit: 657da22f0229b978b7bf4e4d476f59f17f6a175f
    guix a43c3b1
        repository URL: https://git.savannah.gnu.org/git/guix.git
        branch: master
        commit: a43c3b1818db4860ec81bf867b1e53b418a0acec