Port to swiftly 1.0.0

[ADKaster]

Is your feature request related to a problem? Please describe.

Swiftly 1.0.0 has downloads available for each swift.org release, as well as branch snapshots

Describe the solution you'd like

Use the latest swiftly in the action to allow installing even more up to date toolchains

https://www.swift.org/install/linux/
https://www.swift.org/install/macos/

[fwal]

Couldn't agree more! Happy to announce that the process have begun 🥳 #710

[ADKaster]

That's great! If you need any inspiration, I set this up manually for my project here: https://github.com/LadybirdBrowser/ladybird/blob/64577ad7047a91ca617cb23c5cc3832ddd4c47f7/.github/actions/setup/action.yml#L63-L105