Set experimental features for recursive-nix

[jonahbron]

I'm trying to test a Nix expression that depends on recursive-nix. I've tried enabling it on my system and with a command-line argument.

nix-unit --flake '.#tests.aarch64-linux' --extra-experimental-features recursive-nix

# OR

nix-unit --flake '.#tests.aarch64-linux' --option experimental-features "recursive-nix flakes"

But the test fails with this error:

error:
       … while calling the 'readFile' builtin
         at /nix/store/mylmyn3ampxjjr957ca7rq3p03dz6w3g-source/lib/templatize_test.nix:19:14:
           18|       in {
           19|       expr = readFile (invokeTemplate template (writeText "file.txt" "hello"));
             |              ^
           20|       expected = "foo\n''''\nhello";

       … while realising the context of path '/nix/store/2n6b5w6zpc12jdaqf0dr0my2qya1zb35-applied-template'

       error: a 'aarch64-linux' with features {recursive-nix} is required to build '/nix/store/j09lvhx7ccnv5fjg3b8hvrnwd4y7fa36-applied-template.drv', but I am a 'aarch64-linux' with features {benchmark, big-parallel, nixos-test, uid-range

At the bottom it's clear that the feature is not enabled. How can I enable it?