Fix bug permission denied on ci build with external cache

What does this MR do?

Fixes permission denied bug, during build with external cache. (VirtualBox and Parallels)

Why was this MR needed?

I want to use the external cache in the build. But build failed, with permission denied message.

Running with gitlab-ci-multi-runner 1.6.1 (c52ad4f)
Using VirtualBox version 5.1.6r110634 executor...
Restoring VM from snapshot...
Starting VM...
Waiting VM to become responsive...
Starting SSH command...
Running on runner via ml115...
Cloning repository...
Cloning into '/home/buildagent/builds/masakura/gitlab-runner-external-cache-bug'...
Checking out b269f1de as master...
Checking cache for build/master...
WARNING: open ../../../caches/masakura/gitlab-runner-external-cache-bug/build/master/cache081133206: permission denied 
FATAL: open ../../../caches/masakura/gitlab-runner-external-cache-bug/build/master/cache.zip: permission denied 
ERROR: Build failed: Process exited with: 1. Reason was:  ()

• passed: Ommit build cache
• failed: Enable build cache with gitlab-runner (v1.6.1)
• passed: Enable build cache with gitlab-runner (this merge request version)

Are there points in the code the reviewer needs to double check?

no

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?