ci: scope conan cache key by build type in build_debug.yml

[leoparente]

Summary

• The Debug code-coverage and build jobs in build_debug.yml used the same conan cache key as the Release pipeline (conan-${{ runner.os }}-amd64-<hash>).
• actions/cache@v5 saves only the first cache populated under a given key; the permissive restore-keys: conan-${{ runner.os }}-amd64- fallback then let Debug runs restore a Release-populated build/p/ that lacks debug-built dependencies.
• Result: every Debug CI run rebuilt third-party deps from source instead of pulling them from cache.
• Fix: tag the Debug primary key and restore-keys with -debug- so the two pipelines maintain independent caches.

Test plan

• First post-merge Debug run: expect a cache miss (new key), full conan build.
• Subsequent Debug runs: expect a cache hit and no source rebuild of conan packages.
• Release (build-develop.yml) cache behavior unchanged.

[leoparente]

@codex review

[github-actions]

LCOV of commit 7045a0a during Debug Builds #120

  lines......: 81.1% (13546 of 16698 lines)
  functions..: 72.4% (1396 of 1929 functions)
  branches...: no data found

Files changed coverage rate: n/a

Full coverage report