nix: add the python env to check inputs

The tests now require Python so the env should be added to the check
inputs, otherwise the build fails.

flake.nix

@@ -42,6 +42,8 @@
             gmp
           ];
 
+          checkInputs = [pythonEnv];
+
           # Use x86-64-v3 (AVX2) for a portable binary; override with ARCH=native for local builds
           ARCH = "x86-64-v3";