cli: prefer detected python executable instead of harcoded one

In ArchLinux, python means python3

v2: Don't change Windows behavior -- Jose.
diff --git a/cli/cli_diff.cpp b/cli/cli_diff.cpp
index 6aec76f..daeccd3 100644
--- a/cli/cli_diff.cpp
+++ b/cli/cli_diff.cpp
@@ -47,7 +47,7 @@
     os::String command = find_command();
 
     char *args[4];
-    args[0] = (char *) "python";
+    args[0] = (char *) APITRACE_PYTHON_EXECUTABLE;
     args[1] = (char *) command.str();
     args[2] = (char *) "--help";
     args[3] = NULL;
@@ -65,7 +65,7 @@
     os::String apitracePath = os::getProcessName();
 
     std::vector<const char *> args;
-    args.push_back("python");
+    args.push_back(APITRACE_PYTHON_EXECUTABLE);
     args.push_back(command.str());
     args.push_back("--apitrace");
     args.push_back(apitracePath.str());