Use arch=amd64 in APT sources to avoid warnings on multiarch systems.

[alexrp]

I do development for various architectures, so I have my Ubuntu system set up to use multiarch. Since Zulip does not specify an architecture in its APT sources, I get this during apt update:

N: Skipping acquire of configured file 'main/binary-ppc64el/Packages' as repository 'https://download.zulip.com/desktop/apt stable InRelease' doesn't support architecture 'ppc64el'
N: Skipping acquire of configured file 'main/binary-armhf/Packages' as repository 'https://download.zulip.com/desktop/apt stable InRelease' doesn't support architecture 'armhf'
N: Skipping acquire of configured file 'main/binary-s390x/Packages' as repository 'https://download.zulip.com/desktop/apt stable InRelease' doesn't support architecture 's390x'
N: Skipping acquire of configured file 'main/binary-arm64/Packages' as repository 'https://download.zulip.com/desktop/apt stable InRelease' doesn't support architecture 'arm64'
N: Skipping acquire of configured file 'main/binary-riscv64/Packages' as repository 'https://download.zulip.com/desktop/apt stable InRelease' doesn't support architecture 'riscv64'

The fix is simply to set [arch=amd64] so APT knows to only fetch packages for that architecture instead of all enabled architectures.

See also: https://github.com/zulip/zulip/pull/30310