Add nixos configuration for the second testagent machine

[joinemm]

• Refactored the testagent configuration.
• Updated rev of nixos-anywhere as it was having issues with most recent version of nix on my laptop.
• Removed unnecessary suspend blockers because now that there is no GDM the machine is headless and doesn't suspend anyway.
• Added testagent-dev config with disko module, and installed using nixos-anywhere (inv install).
• testagent-dev does not do anything yet and has no jenkins agent configured.

Note for future re-installs that the hardware has issues with kexec (was unable to fix), so I had to boot nixos installer iso before connecting to it with nixos-anywhere to avoid kexec.

[mkaapu]

Note for future re-installs that the hardware has issues with kexec (was unable to fix), so I had to boot nixos installer iso before connecting to it with nixos-anywhere to avoid kexec.

We probably have some sort of documentation about our test HW and/or how to deploy these test machines. Would be worth to document these kind of exceptions and possible future development needs there.