CI: Fix clang-tidy installation on Ubuntu

.github/workflows/main.yml

@@ -63,6 +63,11 @@ jobs:
       - name: Install (Ubuntu)
         if: runner.os == 'Linux'
         run: |
+          # TODO: Remove once https://github.com/actions/virtual-environments/issues/1536 is resolved.
+          wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key 2>/dev/null | sudo apt-key add -
+          sudo add-apt-repository 'deb http://apt.llvm.org/focal llvm-toolchain-focal-10 main' -y
+          sudo apt-get update -q
+
           sudo apt-get install -y --no-install-recommends ninja-build clang-tidy-10
           sudo ln -s /usr/bin/clang-tidy-10 /usr/local/bin/clang-tidy