{CI} pytest: --boxed argument is deprecated, use --forked instead

[wangzelin007]

The latest pytest-xdist 3.0.2 (2022-10-25) has removed --boxed argument.
But we still need this functionality, install pytest-forked separately and use --forked instead.
Error log:

ERROR: usage: __main__.py [options] [file_or_dir] [file_or_dir] [...]
__main__.py: error: unrecognized arguments: --boxed
  inifile: None
  rootdir: /mnt/vss/_work/1/s/src/azure-cli-core

Test screenshot:

[jiasli]

This comment is no longer true.

https://pytest-xdist.readthedocs.io/en/latest/changelog.html#pytest-xdist-2-5-0-2021-12-10

#468: The --boxed command line argument is deprecated. Install pytest-forked and use --forked instead. pytest-xdist 3.0.0 will remove the --boxed argument and pytest-forked dependency.