flaky test: `content digest sha256:xxx: not found` in CI

[yankay]

Description

There are often some test errors in the CI,

It seems the image is not pulled.

Log:

2023-06-23T10:00:28.6954646Z === RUN   TestRunCustomRootfs
2023-06-23T10:00:28.6956107Z     container_run_linux_test.go:57: assertion failed: res.ExitCode is not exitCode: time="2023-06-23T10:00:28Z" level=fatal msg="failed to get reader: content digest sha256:ca3cd42a7c9525f6ce3d64c1a70982613a8235f0cc057ec9244052921853ef15: not found"
2023-06-23T10:00:28.6956612Z         
2023-06-23T10:00:28.6956905Z --- FAIL: TestRunCustomRootfs (0.30s)
2023-06-23T10:00:28.6957247Z FAIL github.com/containerd/nerdctl/cmd/nerdctl.TestRunCustomRootfs (0.30s)
2023-06-23T10:03:31.0098479Z === RUN   TestImageConvertNydus
2023-06-23T10:03:31.0099449Z     image_convert_linux_test.go:41: assertion failed: res.ExitCode is not exitCode: time="2023-06-23T10:03:31Z" level=fatal msg="content digest sha256:ca3cd42a7c9525f6ce3d64c1a70982613a8235f0cc057ec9244052921853ef15: not found"
2023-06-23T10:03:31.0099954Z         
2023-06-23T10:03:31.0100273Z --- FAIL: TestImageConvertNydus (0.37s)
2023-06-23T10:03:31.0100643Z FAIL github.com/containerd/nerdctl/cmd/nerdctl.TestImageConvertNydus (0.37s)
2023-06-23T10:03:31.4190709Z === RUN   TestImageConvertZstdChunked
2023-06-23T10:03:31.4197555Z     image_convert_test.go:52: assertion failed: res.ExitCode is not exitCode: time="2023-06-23T10:03:31Z" level=fatal msg="content digest sha256:ca3cd42a7c9525f6ce3d64c1a70982613a8235f0cc057ec9244052921853ef15: not found"
2023-06-23T10:03:31.4203446Z         
2023-06-23T10:03:31.4209424Z --- FAIL: TestImageConvertZstdChunked (0.41s)
2023-06-23T10:03:31.4215481Z FAIL github.com/containerd/nerdctl/cmd/nerdctl.TestImageConvertZstdChunked (0.41s)
2023-06-23T10:03:31.7625504Z PASS github.com/containerd/nerdctl/cmd/nerdctl.TestImageInspectContainsSomeStuff (0.30s)

Describe the results you received and expected

The CI can pass successfully