Make ca-derivations require a `ca-derivations` machine feature, and
ca-aware builders expose it.
That way, a network of builders can mix ca-aware and non-ca-aware
machines, and the scheduler will send them in the right place.
This seems more correct. It also means one can specify the features a
store should support with --store and remote-store=..., which is useful.
I use this to clean up the build remotes test.
Disable various tests if the kernel doesn't support unprivileged user
namespaces (e.g. Arch Linux disables them) or disable them via a sysctl
(Debian, Ubuntu).
Fixes#1521Fixes#1625
In particular, drop the "build-" and "gc-" prefixes which are
pointless. So now you can say
nix build --no-sandbox
instead of
nix build --no-build-use-sandbox
