Set matching CC alongside CXX in Linux CI matrix

[hexonal]

Fixes #4858

Problem

The Linux CI matrix deliberately varies matrix.cxx across old and new compilers (g++-4.9, clang++-3.6, clang++-20, etc.) to test compatibility, but the Configure step only sets CXX. With no CC, CMake's C compiler detection falls back to whatever default happens to be on the runner (as reported: clang++-3.6 for C++, but GNU 11.4.0 detected for C) — completely unrelated to the compiler version the job is actually meant to be testing.

Fix

Add a small step before Configure that derives CC from the same matrix.cxx value each job already installs a matching compiler for (clang++-N → clang-N, g++-N → gcc-N):

cc=${{matrix.cxx}}
cc=${cc/clang++/clang}
cc=${cc/g++/gcc}
echo "CC=$cc" >> "$GITHUB_ENV"

Testing

This is a CI-config-only change, so the real test is the workflow run itself — I checked every distinct matrix.cxx value used in this file (g++-4.9, g++-11, g++-13, g++-14, clang++-3.6, clang++-11, clang++-14, clang++-20) against the corresponding install/package steps in the same file to confirm a matching gcc-N/clang-N binary is actually installed for each before relying on it (e.g. the "Install GCC 4.9" step explicitly dpkg -is a gcc-4.9 package, "Install Clang 3.6" explicitly installs clang-3.6, etc.) — the two entries without an explicit install step (g++-13/g++-14 on ubuntu-24.04, one clang++-14 variant) rely on the runner image's default toolchain, where the C and C++ drivers for the same version are always installed together. I also verified the string-substitution logic locally in bash against all 8 values (clang++-3.6 correctly does not get double-substituted by the g++→gcc replacement, since the clang++→clang substitution runs first and already consumes the trailing g++ substring), and validated the YAML with ruby -ryaml. I can't run the actual matrix locally, so I'll be watching this PR's own CI run closely.

[hexonal]

The clang++-3.6/g++-4.9 jobs failed on this run, but not because of this PR's change. The actual error in the clang++-3.6 jobs is wget failing to download the old compiler .debs from launchpadlibrarian.net with repeated ERROR 503: Service Unavailable — an external package-mirror outage, unrelated to the CC/CXX selection logic this PR touches (the new "Select matching C compiler" step itself ran fine, correctly resolving CC=gcc-4.9/CC=clang-3.6). The g++-4.9 jobs show as cancelled rather than failed, which is the matrix's default fail-fast behavior kicking in once the clang++-3.6 jobs errored.

I don't have permission to re-run just the failed jobs (not a repo admin) — happy to push an empty commit to retrigger if useful, or this should resolve on its own next time CI runs against this branch.

[vitaut]

I triggered a re-run.