checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
Also enable unity build because it makes LTO more efficient - we don't
need the non-unity fix then.
Also move the docs into their own output because most people using
Icinga in a cluster only need the docs on one machine.
Also clean the vim output to use `runCommand`