Remove Github CI matrix

Due to the limitations explained in
8274e77 we won't be able to use that
for a while anyway.

.github/workflows/ci.yml

@@ -2,13 +2,7 @@ name: Continuous Integration
 on: [push, pull_request]
 jobs:
   build:
-    strategy:
-      matrix:
-        cxx: [gcc]
-        # cxx: [gcc, clang]
     runs-on: ubuntu-latest
-    env:
-      NIX_STDENV: ${{ matrix.cxx }}
     steps:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@v27