Specify "[arch=amd64]" in Apt sources

[threepistons]

To stop the notice at the end of sudo apt update output.

root@mypc:~# apt update
# [snip]                      
Hit:7 https://packages.microsoft.com/ubuntu/18.04/prod insiders-slow InRelease                                                  
# [snip]
Fetched 200 kB in 2s (106 kB/s)                
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up-to-date.
N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'https://packages.microsoft.com/ubuntu/18.04/prod insiders-slow InRelease' doesn't support architecture 'i386'

See https://askubuntu.com/questions/430705/how-to-use-apt-get-to-download-multi-arch-library for how.

Priority: low, cosmetic enhancement.