Added default fall-back when CARGO_HOME is not set for clean-cargo-cache

[terracotaPie]

Added default fall-back when CARGO_HOME is not set.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #19823 (github issue number if applicable).

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This change is 

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @mbrubeck (or someone else) soon.

[highfive]

Heads up! This PR modifies the following files:

• @wafflespeanut: python/servo/bootstrap_commands.py

[terracotaPie]

Let me know if I should fix anything :) Edit: it seems like some of the tests are failing, namely on windows:

WindowsError: [Error 3] The system cannot find the path specified: u'C:\\Users\\appveyor\\.cargo\\git\\db\\*.*'

Is it because .cargo location on windows is in the appdata?

More edit: I see that other PR's have similar issue

[terracotaPie]

It seems after #19831 is fixed this should work fine. It seems jdm already proposed a pull request

[tigercosmos]

@terracotaPie Seems not work?

AppVeyor Log

mach clean-cargo-cache --keep 2 --force
Error running mach:
    ['clean-cargo-cache', '--keep', '2', '--force']
The error occurred in the implementation of the invoked mach command.
This should never occur and is likely a bug in the implementation of that
command. Consider filing a bug for this issue.
If filing a bug, please include the full output of mach, including this error
message.
The details of the failure are as follows:
WindowsError: [Error 3] The system cannot find the path specified: u'C:\\Users\\appveyor\\.cargo\\git\\db\\*.*'
  File "C:\projects\servo\python\servo\bootstrap_commands.py", line 239, in clean_cargo_cache
    git_db_list = filter(lambda f: not f.startswith('.'), os.listdir(git_db_dir))
Command exited with code 1

[terracotaPie]

@tigercosmos other PRs have the same error. There is a fix which I will rebase as soon as it gets into master #19832

[tigercosmos]

@terracotaPie Ah! Sorry. I misunderstand that this PR can also fix that issue.

[terracotaPie]

@tigercosmos No worries. I'll wait for the PR to be in master and then rebase. Would you mind reviewing then?

[jdm]

@bors-servo r+
This change will make it easier to experiment with other possible solutions for the appveyor situation.

[bors-servo]

📌 Commit a8ac8c8 has been approved by jdm

[bors-servo]

⌛ Testing commit a8ac8c8 with merge 06aa339...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, windows-msvc-dev
Approved by: jdm
Pushing 06aa339 to master...