ci: use Intel ifx/icx compilers in Intel CI jobs

[sbryngelson]

Summary

The Intel matrix entries in the test suite install oneAPI and source setvars.sh, which puts Intel compilers on PATH but does not set FC/CC/CXX. Without these, CMake finds gfortran first and the "Intel" CI jobs silently test gfortran instead of ifx.

This PR sets:

• FC=ifx, CC=icx, CXX=icpx — so CMake uses Intel compilers
• MPIFC=mpiifx, MPICC=mpiicx, MPICXX=mpiicpx — so find_package(MPI) picks up Intel MPI wrappers

Test plan

• CI: Github (ubuntu, mpi, reldebug, true) builds with IntelLLVM compiler identification
• CI: Github (ubuntu, mpi, no-debug, true) builds and tests pass with ifx

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 336487c and 31f403b.

📒 Files selected for processing (2)

• .github/workflows/test.yml
• CMakeLists.txt

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📝 Walkthrough

Walkthrough

The pull request extends the GitHub Actions test workflow to add support for Intel oneAPI compilers. The Ubuntu (Intel) setup step now installs the additional intel-oneapi-compiler-dpcpp-cpp package and explicitly sets environment variables for Intel Fortran, C, and C++ compilers along with their MPI variants (ifx, icx, icpx, mpiifx, mpiicx, mpiicpx). A corresponding comment is added to CMakeLists.txt documenting this Intel compiler CI support.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: enabling Intel ifx/icx compilers in CI jobs.
Description check	✅ Passed	The PR description provides clear context, motivation, and test results, though it omits some template sections like explicit type categorization.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.67%. Comparing base (336487c) to head (a83c814).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1342   +/-   ##
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  Coverage   64.67%   64.67%           
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  Files          70       70           
  Lines       18249    18249           
  Branches     1504     1504           
=======================================
  Hits        11803    11803           
  Misses       5491     5491           
  Partials      955      955           

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