Fix apt-get update warnings by specifying the architecture for the repos

[anx-ag]

Getting rid of these messages during apt-get update:

# apt-get update N: Skipping acquire of configured file 'nginx/binary-i386/Packages' as repository 'https://nginx.org/packages/ubuntu focal InRelease' doesn't support architecture 'i386' N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'https://deb.packages.mattermost.com focal InRelease' doesn't support architecture 'i386' N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'http://apt.postgresql.org/pub/repos/apt focal-pgdg InRelease' doesn't support architecture 'i386'

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