Ensure self-hosted runners have `nix` installed

[mmlb]

We've learned that we can't run rooty things which means we can't installed needed binaries if they're missing. If only there was a package manager that we could use within another linux os ... ;).
If we had nix installed then we'd be able to re-work the CI to make use of it and then the self-hosted build runner would get much more use.

Expected Behaviour

Ability to use binaries not currently installed (or at at different versions) on the self-hosted build runners.

Current Behaviour

Can't build boots on the self hosted runner due to missing binaries.

Possible Solution

Ensure nix is available to the CI jobs.

[mmlb]

looks like this is done