CachedDownloadResult: Include store path

Also, make fetchGit and fetchMercurial update allowedPaths properly.

(Maybe the evaluator, rather than the caller of the evaluator, should
apply toRealPath(), but that's a bigger change.)

(cherry picked from commit 5c34d665386f4053d666b0899ecca0639e500fbd)

src/libstore/download.hh

@@ -43,6 +43,9 @@ struct DownloadResult
 
 struct CachedDownloadResult
 {
+    // Note: 'storePath' may be different from 'path' when using a
+    // chroot store.
+    Path storePath;
     Path path;
     std::optional<std::string> etag;
 };