buildGoModule: warn about flags only when using provided buildPhase

Warn about buildFlags and ldflags only when using buildPhase provided by
buildGoModule.

This allows developers to use buildFlags in custom buildPhase.

pkgs/build-support/go/module.nix

···
       runHook postConfigure
     '');
 
-    buildPhase = args.buildPhase or (''
+    buildPhase = args.buildPhase or (
+      lib.warnIf (buildFlags != "" || buildFlagsArray != "")
+        "Use the `ldflags` and/or `tags` attributes instead of `buildFlags`/`buildFlagsArray`"
+      lib.warnIf (builtins.elem "-buildid=" ldflags)
+        "`-buildid=` is set by default as ldflag by buildGoModule"
+    ''
       runHook preBuild
 
       exclude='\(/_\|examples\|Godeps\|testdata'
···
     } // meta;
   });
 in
-lib.warnIf (buildFlags != "" || buildFlagsArray != "")
-  "Use the `ldflags` and/or `tags` attributes instead of `buildFlags`/`buildFlagsArray`"
-lib.warnIf (builtins.elem "-buildid=" ldflags) "`-buildid=` is set by default as ldflag by buildGoModule"
 lib.warnIf (builtins.elem "-trimpath" GOFLAGS) "`-trimpath` is added by default to GOFLAGS by buildGoModule when allowGoReference isn't set to true"
 lib.warnIf (lib.any (lib.hasPrefix "-mod=") GOFLAGS) "use `proxyVendor` to control Go module/vendor behavior instead of setting `-mod=` in GOFLAGS"
   package