Shallow clone xeus cpp emscripten build

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

Our emscripten build workflows clone the whole of the xeus-cpp history when all we need is the latest commit. This workflow changes it to a shallow clone of xeus-cpp (may provide a small speed increase)

Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.96%. Comparing base (c9499ae) to head (d1f8555).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #442   +/-   ##
=======================================
  Coverage   70.96%   70.96%           
=======================================
  Files           9        9           
  Lines        3541     3541           
=======================================
  Hits         2513     2513           
  Misses       1028     1028