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

Like recursive-nix, ca-derivations is (partially) a build-time feature. This means that (in a distributed build scenario) a remote builder must have the experimental feature enabled to be able to build a ca derivation.
However (unlike recursive-nix), there's currently no way to tell at a distance whether a builder will support it or not, meaning that the scheduler might sent ca derivations to non-ca-enabled builders (which will miserably fail).

Describe the solution you'd like

Like for recursive-nix, make content-addressed derivations require a ca-derivations system feature (implicitly though), and make the ca-derivation experimental feature add this system feature to the default set.

Describe alternatives you've considered

• Do nothing and require users to manually do the system-feature dance if they need to (possibly adding the requiredSystemFeature at the nixpkgs level, at least as a transitionary measure)
• Abuse the fact that remote builders must be trusted users to force them to use the experimental feature. But that sounds rather bad, and wouldn't work with older Nix.