Don't apply the CPU affinity hack to nix-shell (and other Perl programs)

As discovered by Todd Veldhuizen, the shell started by nix-shell has
its affinity set to a single CPU.  This is because nix-shell connects
to the Nix daemon, which causes the affinity hack to be applied.  So
we turn this off for Perl programs.

perl/lib/Nix/Store.xs

@@ -20,6 +20,7 @@ void doInit()
     if (!store) {
         try {
             settings.processEnvironment();
+            settings.lockCPU = false;
             store = openStore();
         } catch (Error & e) {
             croak(e.what());