CI: Add build for linux/arm64

[iainlane]

This builds and pushes a multi-arch Docker manifest for amd64 and arm64.

We use Docker's provided action to do this using qemu emulation, and switch to using this to push the images for us.

Split the build/test of the xk6 binary out to a separate job too, so that can also be run on arm64.

[iainlane]

Example runs where I had hacked the action to work on pushes to iainlane/*:

• master
• tag (gets latest and 1.0.0, the v is dropped)
• rc (gets 1.0.0-rc.1 but not latest)

Expand "Set up tags" -> "Docker tags" to see what each run tagged.

See the resulting pushed container images:

• GHCR
• Docker Hub

[pablochacin]

Closing due to staleness. If this is required we need to open a new PR aligned with the current vision of the tooling ecosystem