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 79.93%. Comparing base (4fe3ade) to head (3323fe9).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8789   +/-   ##
========================================
  Coverage    79.93%   79.93%           
========================================
  Files         1698     1698           
  Lines       187698   187698           
  Branches        73       73           
========================================
  Hits        150034   150034           
  Misses       37664    37664           

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