GC: Don't delete own temproots file

Since file locks are per-process rather than per-file-descriptor, the
garbage collector would always acquire a lock on its own temproots
file and conclude that it's stale.

src/libstore/local-store.hh

@@ -59,7 +59,6 @@ private:
         SQLiteStmt stmtQueryValidPaths;
 
         /* The file to which we write our temporary roots. */
-        Path fnTempRoots;
         AutoCloseFD fdTempRoots;
     };
 
@@ -75,6 +74,8 @@ public:
     const Path reservedPath;
     const Path schemaPath;
     const Path trashDir;
+    const Path tempRootsDir;
+    const Path fnTempRoots;
 
 private: