Key CUDA, Python, and coverage ccaches on the CPU

[zfergus]

Description

Key the ccache in the CUDA, Python, and coverage workflows on the runner's CPU, as continuous.yml already does.

• FindSIMD builds with -march=native, but ccache hashes the flag string, not the CPU it expands to.
• These three workflows restored one fixed cache (e.g. Linux-Debug-cache) onto whichever CPU the next runner had.
• Objects built on one CPU then mix with objects compiled fresh on another, and xsimd::default_arch can differ between them.
• This is the suspected cause of the ipc_toolkit_tests link failure in #265: undefined references to xsimd::batch<…, avx512vnni<avx512vbmi2>> that main's CUDA run did not hit. It is unconfirmed.

Changes

• cuda.yml, coverage.yml: add a Linux CPU fingerprint step (hash of model name in /proc/cpuinfo).
• python.yml: add the same step for Linux, macOS, and Windows, copied from continuous.yml.
• Cache keys gain ${{ env.CPU_KEY }}. Old keys (Linux-Debug-cache, coverage-cache, …) are no longer restored.

Impact

• The first run on each CPU model starts with a cold cache, so expect one slow CUDA build per runner type.
• Keys are one entry per (OS, config, CPU), so the cache list grows and old entries age out.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Workflow YAML parses (yaml.safe_load on all three files).
• CUDA, Python, and coverage jobs on this PR run with the new keys. The first run is cold by design.

Checklist

• I have commented my code, particularly in hard-to-understand areas

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.92%. Comparing base (0985885) to head (adcb0d0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #267   +/-   ##
=======================================
  Coverage   96.92%   96.92%           
=======================================
  Files         194      194           
  Lines       17348    17349    +1     
  Branches      943      943           
=======================================
+ Hits        16814    16815    +1     
  Misses        534      534           

Flag	Coverage Δ
unittests	96.92% <ø> (+<0.01%)	⬆️

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