Use forked version of firecracker

[Callisto13]

Firecracker's macvtap branch is nudging 500 commits behind main, and we are not sure when that feature will be merged or even what support will be available.

To avoid any nasty surprises, we are going to:

• Fork firecracker (Richard already has a fork, so we could use that if we like)
• Ensure that the forked macvtap branch is kept up with both upstream main and the upstream feature branch (this can either be an automated cron job, or a manual process)

[jmickey]

I have created ticket weaveworks/corp#2267 for this.

[richardcase]

This is still blocked on the corp ticket

[Callisto13]

I am gonna carry on with this.

• get tests running
• update docs which reference the previous branch/binary
• create action which regularly updates the fork and creates a binary (store binary where? maybe just have a fork release?) (update when on upstream release or weekly from main?)
• use that binary in the nightly e2es
• slack alert of new bin
• see if this is possible Investigate: why can we not set only one of initrd or root volume? #219