clamav: Fix check failure of clamscan no tests

It seems like clamav uses `pytest` as a test runner preferentially, then
falls back to `unittest`. For some reason `unittest` isn't discovering
the tests inside clamav/unit_test/clamscan, and instead exitting with
`sys.exit(_NO_TESTS_EXITCODE)`.

I have checked actual `clamscan` tests are being run using
`nix develop .#clamscan`, `genericBuild`, `checkPhase` then manually
running `ctest -V`.

Fixes https://github.com/NixOS/nixpkgs/issues/325510

pkgs/tools/security/clamav/default.nix

@@ -33,6 +33,10 @@ stdenv.mkDerivation rec {
 
   doCheck = true;
 
+  checkInputs = [
+    python3.pkgs.pytest
+  ];
+
   meta = with lib; {
     homepage = "https://www.clamav.net";
     description = "Antivirus engine designed for detecting Trojans, viruses, malware and other malicious threats";