Fix data race in gitlab source cache package

The following discussion from !246 (merged) should be addressed:

• @nolith started a discussion: (+1 comment)

  Here the race is deep inside the package

  we make use of global variables and this test will alter retrievalTimeout when a goroutine from the previous one is still running.

  This will require extra effort to fix, but I think we should add the race detector now.

   ==================
   WARNING: DATA RACE
   Write at 0x000000e0d4f0 by goroutine 46:
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.TestResolve.func9()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/cache_test.go:247 +0x3e
     testing.tRunner()
         /usr/local/go/src/testing/testing.go:827 +0x162
   Previous read at 0x000000e0d4f0 by goroutine 44:
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.(*Retriever).Retrieve()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/retriever.go:22 +0x8d
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.(*Entry).retrieveWithClient()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/entry.go:92 +0xc5
   Goroutine 46 (running) created at:
     testing.(*T).Run()
         /usr/local/go/src/testing/testing.go:878 +0x659
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.TestResolve()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/cache_test.go:246 +0x1d5
     testing.tRunner()
         /usr/local/go/src/testing/testing.go:827 +0x162
   Goroutine 44 (running) created at:
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.(*Entry).Retrieve.func1()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/entry.go:64 +0x67
     sync.(*Once).Do()
         /usr/local/go/src/sync/once.go:44 +0xde
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.(*Entry).Retrieve()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/entry.go:64 +0xad
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.(*Cache).Resolve()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/cache.go:89 +0x1ea
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.TestResolve.func8.1()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/cache_test.go:239 +0x76
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.withTestCache()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/cache_test.go:89 +0x185
     gitlab.com/gitlab-org/gitlab-pages/internal/source/gitlab/cache.TestResolve.func8()
         /builds/nolith/gitlab-pages/internal/source/gitlab/cache/cache_test.go:238 +0xc7
     testing.tRunner()
         /usr/local/go/src/testing/testing.go:827 +0x162
   ==================