fix: Pre-enable ca-derivations in tests

This way we don't run into any race conditions when individual tests
enable the feature.
Specifically this affects the initialization of our default store for
testing.
The test suite *should* not be all that sensitive to the environment,
but that's some future work to make sure.

rust/nci.nix

@@ -56,6 +56,10 @@
 
               echo "Configuring relocated store at $NIX_REMOTE..."
 
+              # Create nix.conf with experimental features enabled
+              mkdir -p "$NIX_CONF_DIR"
+              echo "experimental-features = ca-derivations flakes" > "$NIX_CONF_DIR/nix.conf"
+
               # Init ahead of time, because concurrent initialization is flaky
               ${
                 # Not using nativeBuildInputs because this should (hopefully) be