Update Emscripten instructions to match xeus-cpp in where dependencies are installed from + mount tag files (no longer preloaded in xeus-cpp)

[mcbarton]

Description

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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 77.70%. Comparing base (3d3c82d) to head (35052db).
⚠️ Report is 72 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #618      +/-   ##
==========================================
+ Coverage   77.66%   77.70%   +0.03%     
==========================================
  Files           9        9              
  Lines        3743     3749       +6     
==========================================
+ Hits         2907     2913       +6     
  Misses        836      836              

see 1 file with indirect coverage changes
see 1 file with indirect coverage changes

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[mcbarton]

This PR is ready for review now. With the latest change to xeus-cpp, the tagfiles are mounted to the jupyter lite build instead of preloaded. Without that change to CppInterOps ci, part of the deployment will be broken after the next nightly build.

[vgvassilev]

Looks good to me but let's have another pair of eyes.