Fix no export subcmd panic on mac

[An-DJ]

Relevant Issue (if applicable)

If there are Issues related to this PullRequest, please list it.

Fix panic caused by no export subcmd on mac in issue #1340

Details

Please describe the details of PullRequest.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1429 (c7aeefd) into master (d2fcfcd) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1429      +/-   ##
==========================================
- Coverage   46.46%   46.44%   -0.02%     
==========================================
  Files         123      123              
  Lines       38643    38643              
  Branches    38643    38643              
==========================================
- Hits        17954    17948       -6     
- Misses      19716    19722       +6     
  Partials      973      973              

Files Changed	Coverage Δ
src/bin/nydus-image/main.rs	41.62% <0.00%> (ø)

... and 3 files with indirect coverage changes

[imeoer]

Thanks for the fixup! Maybe give the warning to tell users we can't support export subcommand for macos here?

[An-DJ]

Actually if the user runs export on macos, it cannot be recognized.

% ./target/debug/nydus-image export
error: The subcommand 'export' wasn't recognized

Usage: nydus-image [OPTIONS] [COMMAND]

For more information try '--help'

IMHO this error msg is enough, isn't it ? :)

[An-DJ]

Also export has been removed from the available subcmd on macos:

% ./target/debug/nydus-image -h
Build, analyze, inspect or validate RAFS filesystems/Nydus accelerated container images

Usage: nydus-image [OPTIONS] [COMMAND]

Commands:
  create     Create RAFS filesystems from directories, tar files or OCI images
  chunkdict  deduplicate RAFS filesystem metadata
  merge      Merge multiple bootstraps into a overlaid bootstrap
  check      Validate RAFS filesystem metadata
  inspect    Inspect RAFS filesystem metadata in interactive or request mode
  stat       Generate statistics information for RAFS filesystems
  compact    (experimental)Compact specific nydus image, remove unused chunks in blobs, merge small blobs
  unpack     Unpack a RAFS filesystem to a tar file
  help       Print this message or the help of the given subcommand(s)

Options:
  -L, --log-file <log-file>    Log file path
  -l, --log-level <log-level>  Log level: [default: info] [possible values: trace, debug, info, warn, error]
  -h, --help                   Print help information
  -V, --version                Print version information

[imeoer]

Okay, maybe some macos users ask why not have export subcommand on his mac, but I think it's also okay for me.