build: move needsHashRewrite initialization to startBuilder

The value of useChroot is not set yet in the constructor, resulting in
hash rewriting being enabled in certain cases where it should not be.

Fixes #2801

src/libstore/build.cc

@@ -997,13 +997,6 @@ DerivationGoal::DerivationGoal(const Path & drvPath, const StringSet & wantedOut
     , wantedOutputs(wantedOutputs)
     , buildMode(buildMode)
 {
-#if __linux__
-    needsHashRewrite = !useChroot;
-#else
-    /* Darwin requires hash rewriting even when sandboxing is enabled. */
-    needsHashRewrite = true;
-#endif
-
     state = &DerivationGoal::getDerivation;
     name = (format("building of '%1%'") % drvPath).str();
     trace("created");
@@ -1852,6 +1845,13 @@ void DerivationGoal::startBuilder()
         #endif
     }
 
+#if __linux__
+    needsHashRewrite = !useChroot;
+#else
+    /* Darwin requires hash rewriting even when sandboxing is enabled. */
+    needsHashRewrite = true;
+#endif
+
     /* If `build-users-group' is not empty, then we have to build as
        one of the members of that group. */
     if (settings.buildUsersGroup != "" && getuid() == 0) {