`"is-main-source": false` is treated as main source

[adil192]

In https://github.com/flathub/com.adilhanney.saber/pull/100, I set is-main-source to false for the x86_64 source (following https://github.com/flathub/flatpak-external-data-checker/issues/379#issuecomment-1806862390).

But https://github.com/flathub/com.adilhanney.saber/pull/101 created a pull request for just the x86_64 source.

I'm guessing this is because is-main-source is present even though it is false.

[wjt]

To only open PRs when at least one important source is updated, first set require-important-update to true in flathub.json.

• https://github.com/flathub/flatpak-external-data-checker#selectively-submitting-prs