Fix wasm build instructions

[mcbarton]

Description

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

This PR fixes issues in the wasm build instructions. The first is that we cd into home to install and activate emsk, then try and activate yml file in xeus-cpp repo, but we are no longer in it. The second is that the micromamba create command doesn't work unless you point it towards the channel needed.

Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.72%. Comparing base (0a9e71e) to head (8ea4937).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #230   +/-   ##
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  Coverage   80.72%   80.72%           
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  Files          19       19           
  Lines         970      970           
  Branches       93       93           
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  Hits          783      783           
  Misses        187      187