fix(IDX): use llvm-clang on Intel macOS (dfinity#3530)

This ensures that the x86-darwin builders use a working c compiler
(llvm-clang) instead of Apple's, which causes build failures.

Co-authored-by: Bas van Dijk <[email protected]>

Author: nmattia

.github/workflows-source/ci-main.yml

@@ -189,6 +189,8 @@ jobs:
         run: |
           echo "/usr/local/bin" >> $GITHUB_PATH
           echo "$HOME/.cargo/bin:" >> $GITHUB_PATH
+          # use llvm-clang instead of apple's
+          echo "CC=/usr/local/opt/llvm/bin/clang" >> "$GITHUB_ENV"
       - name: Run Bazel Test Darwin x86-64
         id: bazel-test-darwin-x86-64
         uses:  ./.github/actions/bazel-test-all/

.github/workflows/ci-main.yml

@@ -181,6 +181,8 @@ jobs:
         run: |
           echo "/usr/local/bin" >> $GITHUB_PATH
           echo "$HOME/.cargo/bin:" >> $GITHUB_PATH
+          # use llvm-clang instead of apple's
+          echo "CC=/usr/local/opt/llvm/bin/clang" >> "$GITHUB_ENV"
       - name: Run Bazel Test Darwin x86-64
         id: bazel-test-darwin-x86-64
         uses: ./.github/actions/bazel-test-all/