CI setup

[yim-lee]

Motivation: swift-openapi-runtime is private, so it can't be accessed anonymously and CI fails.

Modifications:

• Change to use SSH git URL for swift-openapi-runtime
• Set up SSH access in CI temporarily until swift-openapi-runtime becomes public.

[yim-lee]

@swift-server-bot test this please

[yim-lee]

@swift-server-bot test this please

[yim-lee]

Not sure why the nightly CI keeps timing out.

@swift-server-bot test this please

[yim-lee]

@czechboy0 @simonjbeaumont Feel free to close this PR if the gitconfig approach is preferred. CI should be all set.

[czechboy0]

Thanks, @yim-lee! 🙏