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.

src/libstore/globals.hh

@@ -183,6 +183,9 @@ struct Settings {
        (to prevent them from being GCed). */
     bool envKeepDerivations;
 
+    /* Whether to lock the Nix client and worker to the same CPU. */
+    bool lockCPU;
+
 private:
     SettingsMap settings, overrides;