change to older mac toolchain

Author: mulimoen

.github/workflows/ci.yml

@@ -126,6 +126,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
         with: {submodules: true}
+      - name: Change to older toolchain
+        if: matrix.os == 'macos'
+        run: sudo xcode-select -s "/Applications/Xcode_11.7.app"
       - name: Install Rust (${{matrix.rust}})
         uses: actions-rs/toolchain@v1
         with: {toolchain: '${{matrix.rust}}', profile: minimal, override: true}