Fix Xen CI job by moving to GitHub mirror

[tautschnig]

git://xenbits.xen.org/xen.git appears to be working fine from my local host, but maybe the (plaintext) git protocol is being blocked somewhere. Use GitHub's mirror of the Xen project instead.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (5719027) to head (69d6271).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8789      +/-   ##
===========================================
- Coverage    80.02%   80.01%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188345   188345              
  Branches        73       73              
===========================================
- Hits        150716   150705      -11     
- Misses       37629    37640      +11     

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[Copilot]

Pull request overview

This PR fixes the Xen CI job by switching the Xen repository clone source from the xenbits.xen.org git protocol URL to the GitHub mirror at xen-project/xen, using a specific stable branch with shallow clone instead of cloning the full repo and resetting to a tag.

Changes:

• Replaced the git://xenbits.xen.org/xen.git clone (full clone + reset to RELEASE-4.13.0 tag) with a shallow clone of the stable-4.13 branch from GitHub's mirror.

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