Build specinvoke from source

The one contained in the CPU SPEC ISO is built for x86, and there is no
binary for RISC-V.

pkgs/spec-cpu/mini.nix

@@ -2,6 +2,7 @@
   stdenv
 , spec-cpu
 , spec-cpu-tools
+, specinvoke
 }:
 
 stdenv.mkDerivation rec {
@@ -49,9 +50,8 @@ stdenv.mkDerivation rec {
     mkdir -p $out/bin
     echo -e "#!$SHELL\necho $out" > $out/bin/spec-cpu-mini
     chmod +x $out/bin/spec-cpu-mini
-    cp $(command -v specinvoke) $out/bin
+    cp ${specinvoke}/bin/specinvoke $out/bin
   '';
-  nativeBuildInputs = [ spec-cpu-tools ];
   enableParallelBuilding = false;
   hardeningDisable = [ "all" ];
   dontStrip = true;