go/workspace: fix nix-instantiate exec error typecast

Also skip nix tests on systems without nix.

Change-Id: I4c0069a429df10a496b2651c2506b2d4625d5f43
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1585
Reviewed-by: q3k <q3k@hackerspace.pl>
diff --git a/go/workspace/nix_test.go b/go/workspace/nix_test.go
index acb6d99..d1daf88 100644
--- a/go/workspace/nix_test.go
+++ b/go/workspace/nix_test.go
@@ -2,6 +2,7 @@
 
 import (
 	"context"
+	"os"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -49,6 +50,9 @@
 // TestEvalHscloud nix exercises EvalHscloudNix against
 // //go/workspace/exports.nix.
 func TestEvalHscloudNix(t *testing.T) {
+	if _, err := os.Stat("/nix/store"); err != nil {
+		t.Skip("no /nix/store")
+	}
 	ctx, ctxC := context.WithCancel(context.Background())
 	defer ctxC()