Make use of EMSDK environment variable which comes from sourcing emsdk script

[mcbarton]

Description

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

xeus-cpp instructions create a BUILD_PREFIX and SYSROOT_PATH environment variable. It would be better to just utilise the EMSDK environment variable which comes from when you setup emsdk. Also since we only use SYSROOT_PATH once it makes no sense for it to be an environment variable. We can just put the path directly into the cmake configure stage.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

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

[mcbarton]

The fact the EMSDK environment variable is blank when you get emsdk from Emscripten forge suggests a bug in the Emscripten Forge to me. You get this variable when you setup emsdk using the upstream repo (see https://github.com/compiler-research/CppInterOp/actions/runs/13933008473/job/38994499405#step:9:103) .

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.56%. Comparing base (4ee2748) to head (169fc5c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #280   +/-   ##
=======================================
  Coverage   80.56%   80.56%           
=======================================
  Files          20       20           
  Lines         957      957           
  Branches       88       88           
=======================================
  Hits          771      771           
  Misses        186      186           

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[anutosh491]

The fact the EMSDK environment variable is blank when you get emsdk from Emscripten forge suggests a bug in the Emscripten Forge to me.

Possibly raise an issue on emscripten-forge for the same ?