Use vfork() instead of fork() if available

Hopefully this reduces the chance of hitting ‘unable to fork: Cannot
allocate memory’ errors.  vfork() is used for everything except
starting builders.

src/libstore/local-store.hh

@@ -208,6 +208,8 @@ public:
 
     void markContentsGood(const Path & path);
 
+    void setSubstituterEnv();
+
 private:
 
     Path schemaPath;
@@ -238,6 +240,8 @@ private:
     /* Cache for pathContentsGood(). */
     std::map<Path, bool> pathContentsGoodCache;
 
+    bool didSetSubstituterEnv;
+
     int getSchema();
 
     void openDB(bool create);