apt: less hacky ubuntu sources installation

[tonistiigi]

• Implements xx-apt: add special case for ubuntu repo switch #49 (comment)
• Avoids sid check for riscv on ubuntu

[crazy-max]

LGTM

There is an issue with the fedora:35 image and its repo metadata. This is fixed in 243f662

#16 3.765 (71/71): vim-common-8.2.3642-1.fc35.x86_64.rpm   17 MB/s | 6.7 MB     00:00    
#16 3.766 --------------------------------------------------------------------------------
#16 3.766 Total                                           7.6 MB/s |  18 MB     00:02     
#16 4.361 Running transaction check
#16 4.402 Transaction check succeeded.
#16 4.402 Running transaction test
#16 4.695 The downloaded packages were saved in cache until the next successful transaction.
#16 4.695 You can remove cached packages by executing 'yum clean packages'.
#16 4.701 Error: Transaction test error:
#16 4.701   file /usr/share/vim/vim82/defaults.vim from install of vim-common-2:8.2.3642-1.fc35.x86_64 conflicts with file from package vim-minimal-2:8.2.3404-1.fc35.x86_64
#16 4.701 
#16 ERROR: executor failed running [/bin/sh -c rm -rf /var/cache/yum &&     ln -s /pkg-cache /var/cache/yum &&     yum -y install bats vim]: exit code: 1