DNF variables not replaced for mirror

[supakeen]

When building an image like so:

$ image-builder build --extra-repo 'https://download.copr.fedorainfracloud.org/results/pbrobinson/a64-kernel/fedora-$releasever-$basearch/' minimal-raw-zst
# ...

The depsolve runs succesfully, however the librepo source can't download the RPMs due to (I assume) the variables not being replaced in the mirror definition for it:

$ image-builder manifest --extra-repo 'https://download.copr.fedorainfracloud.org/results/pbrobinson/a64-kernel/fedora-$releasever-$basearch/' minimal-raw-zst | jq .
# ...
            "options": {
                "mirrors": {
                    "79f632b2e58a01595fd49e75eb5244d2bf743e0f3d497308a6e8266c788b4066": {
                        "url": "https://download.copr.fedorainfracloud.org/results/pbrobinson/a64-kernel/fedora-$releasever-$basearch/",
                        "type": "baseurl"
                    },
                    "bd1e99c7372083ac571f60e61fc0dd5a9dc2bef9215afe0980595d4f4cac86f8": {
                        "url": "https://mirrors.fedoraproject.org/metalink?repo=fedora-42&arch=aarch64",
                        "type": "metalink"
                    }
                }
            }
# ...

When manually replacing the variables in the URL passed to --extra-repo the URL is written correctly into the manifest and the build succeeds.