Mark github workflows as generated

[MikailBag]

They can confuse reviewer, because "foo\nbar\nbaz" is considered poor "code" style.

[MikailBag]

github docs: https://help.github.com/en/github/administering-a-repository/customizing-how-changed-files-appear-on-github

[MikailBag]

bors r+

[bors[bot]]

Build succeeded:

• clippy
• codegen
• hack
• py-fmt
• py-test
• style
• udeps
• unit-tests