ci: downgrade x86_64 Apple SDKs to older Xcode

Let's restore support for building with Xcode and the Apple SDKs
installed on macOS 10.15 for maximum compatibility with build
environments (such as GitHub Actions and its 10.15 runner).

The aarch64 targets still build with the latest SDK, as most
builds for this environment occur from macOS 11.

Author: indygreg

.github/workflows/apple.yml

@@ -240,9 +240,9 @@ jobs:
           elif [ "${{ matrix.build.target_triple }}" = "aarch64-apple-ios" ]; then
             export APPLE_SDK_PATH=/Applications/Xcode_12.5.1.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.5.sdk
           elif [ "${{ matrix.build.target_triple }}" = "x86_64-apple-darwin" ]; then
-            export APPLE_SDK_PATH=/Applications/Xcode_12.5.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk
+            export APPLE_SDK_PATH=/Applications/Xcode_12.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
           elif [ "${{ matrix.build.target_triple }}" = "x86_64-apple-ios" ]; then
-            export APPLE_SDK_PATH=/Applications/Xcode_12.5.1.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.5.sdk
+            export APPLE_SDK_PATH=/Applications/Xcode_12.4.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.4.sdk
           else
             echo "unhandled target triple: ${{ matrix.build.target_triple }}"
             exit 1