Unit tests fail using Go 1.19 because of data race

[rdpsin]

Describe the bug
The unit tests inside the soci/ directory fail due a data race. This only happens when using Go 1.19.x, not the previous versions of Go.

Steps To Reproduce

$ go version
go version go1.19 linux/amd64

$ make test 
...
==================
WARNING: DATA RACE
Write at 0x00c001509038 by goroutine 55:
  runtime.racewriterange()
      <autogenerated>:1 +0x29
  internal/poll.(*FD).Pread()
      /usr/local/go/src/internal/poll/fd_unix.go:193 +0x169
  os.(*File).pread()
      /usr/local/go/src/os/file_posix.go:40 +0x335
  os.(*File).ReadAt()
      /usr/local/go/src/os/file.go:136 +0x2de
  io.(*SectionReader).ReadAt()
      /usr/local/go/src/io/io.go:552 +0x205
  github.com/awslabs/soci-snapshotter/soci.ExtractFile.func1()
      /home/ec2-user/soci-snapshotter/soci/ztoc.go:141 +0x1db
  golang.org/x/sync/errgroup.(*Group).Go.func1()
      /home/ec2-user/go/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/errgroup/errgroup.go:57 +0x91

Previous write at 0x00c001509039 by goroutine 56:
  runtime.racewriterange()
      <autogenerated>:1 +0x29
  internal/poll.(*FD).Pread()
      /usr/local/go/src/internal/poll/fd_unix.go:193 +0x169
  os.(*File).pread()
      /usr/local/go/src/os/file_posix.go:40 +0x335
  os.(*File).ReadAt()
      /usr/local/go/src/os/file.go:136 +0x2de
  io.(*SectionReader).ReadAt()
      /usr/local/go/src/io/io.go:552 +0x205
  github.com/awslabs/soci-snapshotter/soci.ExtractFile.func1()
      /home/ec2-user/soci-snapshotter/soci/ztoc.go:141 +0x1db
  golang.org/x/sync/errgroup.(*Group).Go.func1()
      /home/ec2-user/go/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/errgroup/errgroup.go:57 +0x91

Goroutine 55 (running) created at:
  golang.org/x/sync/errgroup.(*Group).Go()
      /home/ec2-user/go/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/errgroup/errgroup.go:54 +0xee
  github.com/awslabs/soci-snapshotter/soci.ExtractFile()
      /home/ec2-user/soci-snapshotter/soci/ztoc.go:135 +0x6eb
  github.com/awslabs/soci-snapshotter/soci.TestDecompress()
      /home/ec2-user/soci-snapshotter/soci/ztoc_test.go:138 +0xf88
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1446 +0x216
  testing.(*T).Run.func1()
      /usr/local/go/src/testing/testing.go:1493 +0x47

Goroutine 56 (running) created at:
  golang.org/x/sync/errgroup.(*Group).Go()
      /home/ec2-user/go/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/errgroup/errgroup.go:54 +0xee
  github.com/awslabs/soci-snapshotter/soci.ExtractFile()
      /home/ec2-user/soci-snapshotter/soci/ztoc.go:135 +0x6eb
  github.com/awslabs/soci-snapshotter/soci.TestDecompress()
      /home/ec2-user/soci-snapshotter/soci/ztoc_test.go:138 +0xf88
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1446 +0x216
  testing.(*T).Run.func1()
      /usr/local/go/src/testing/testing.go:1493 +0x47
==================
--- FAIL: TestDecompress (12.08s)
    testing.go:1319: race detected during execution of test

Expected behavior
All unit tests pass.

Configuration (please complete the following information):

• OS: AL2 (Linux 5.10.130-118.517.amzn2.x86_64)
• Snapshotter Version: c5a2474
• Containerd Version: v1.6.6
• Go Version: go version go1.19 linux/amd64

[vkuzniet]

Fixed with #88