CI: Use clang-12 instead of clang-10 when building fuzzers

oss-fuzz uses clang-12 anyway, so this patch shouldn't be breaking
anything, just letting us use more modern C++ without the CI being
sad.

.github/workflows/cmake.yml

@@ -182,7 +182,14 @@ jobs:
  # === OS SETUP ===
  #
  - name: Install Ubuntu dependencies
- run: sudo apt-get install ninja-build
+ run: |
+ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+ sudo add-apt-repository "deb https://apt.llvm.org/focal/ llvm-toolchain-focal-12 main"
+ sudo apt-get purge -y clang-10
+ sudo apt-get update
+ sudo apt-get install clang-12 ninja-build
+ sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-12 60
+ sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-12 60
  if: ${{ runner.os == 'Linux' }}
  - name: Install macOS dependencies
  run: brew install ninja