ccache: option to allow silencing per-derivation trace

Currently the ccache wrapper prints a trace for each derivation
wrapped. In some contexts this can be distracting; for example, on
nix-output-monitor those traces seem to permanently occupy screen
space that could be used on other information, and they can drown
other important traces.

Add an option, `programs.ccache.trace`, to allow silencing those
traces. For maximum backwards compatibility, default to the current
behavior (`programs.ccache.trace = true`).

nixos/modules/programs/ccache.nix

@@ -38,6 +38,11 @@ in
       default = "nixbld";
       description = "Group owner of CCache directory";
     };
+    trace = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = "Trace ccache usage to see which derivations use ccache";
+    };
   };
 
   config = lib.mkMerge [
@@ -73,7 +78,11 @@ in
         (
           self: super:
           lib.genAttrs cfg.packageNames (
-            pn: super.${pn}.override { stdenv = builtins.trace "with ccache: ${pn}" self.ccacheStdenv; }
+            pn:
+            super.${pn}.override {
+              stdenv =
+                if cfg.trace then builtins.trace "with ccache: ${pn}" self.ccacheStdenv else self.ccacheStdenv;
+            }
           )
         )