inject-checksums is platform-dependent in pythonbundles without arch templates

[surak]

If there's a ext on a pythonbundle which downloads a arch-dependent wheel, it will inject different checksums depending on where you do the injection.

say, you have an extension

    ('%(name)s', %(version)s', {
        'checksums': ['xxx'],
    }),

And this downloads a py3-none-manylinux2014_x86_64.whl in one, and a -py3-linux_aarch64.whl in another, it will checksum the one for its own platform and fail for the other.