ci: use the pre-installed Xcode 12

https://www.appveyor.com/updates/2020/12/27/

Author: cfillion

.appveyor.yml

@@ -65,32 +65,27 @@ for:
       - path: build/reaper_reapack*.so
 
   - matrix: { only: [ appveyor_build_worker_image: macos, appveyor_build_worker_image: macos-mojave ] }
-    install: |-
-      set -e
-
-      brew install boost catch2 > /dev/null
-
-      case $ARCH in
-      arm64)
-        DEPLOY_TARGET=11.0
-        curl -fsSLO https://cfillion.ca/files/.xcode/Command_Line_Tools_for_Xcode_12.2.dmg
-        echo 3911b66247e030c7210940cde4230319058bc5c55a81627b8853dab644092c40 Command_Line_Tools_for_Xcode_12.2.dmg | sha256sum -c
-        hdiutil attach Command_Line_Tools_for_Xcode_12.2.dmg > /dev/null
-        sudo installer -pkg '/Volumes/Command Line Developer Tools/Command Line Tools.pkg' -target /
-        ;;
-      *)
-        DEPLOY_TARGET=10.9
-        export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX10.10.sdk
-        curl -fsSLO https://github.com/phracker/MacOSX-SDKs/releases/download/10.15/MacOSX10.10.sdk.tar.xz
-        echo 608a89db8b4be150a945871230b5ba5d4767a8500bc5fe76ddf10f5cec5ef513 MacOSX10.10.sdk.tar.xz | sha256sum -c
-        sudo tar xf MacOSX10.10.sdk.tar.xz -C /Library/Developer/CommandLineTools/SDKs
-        ;;
-      esac
-
-      sudo xcode-select -s /Library/Developer/CommandLineTools
+    install:
+      - brew install boost catch2 > /dev/null
+
+      - |-
+        case $ARCH in
+        arm64)
+          DEPLOY_TARGET=11.0
+          sudo xcode-select -s /Applications/Xcode-12.3.app
+          ;;
+        *)
+          DEPLOY_TARGET=10.9
+          export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX10.10.sdk
+          curl -fsSLO https://github.com/phracker/MacOSX-SDKs/releases/download/10.15/MacOSX10.10.sdk.tar.xz
+          echo 608a89db8b4be150a945871230b5ba5d4767a8500bc5fe76ddf10f5cec5ef513 MacOSX10.10.sdk.tar.xz | sha256sum -c
+          sudo tar xf MacOSX10.10.sdk.tar.xz -C /Library/Developer/CommandLineTools/SDKs
+          sudo xcode-select -s /Library/Developer/CommandLineTools
+          ;;
+        esac
 
       # don't use LibXml2/SQLite3 from Mono
-      sudo rm -r /Library/Frameworks/Mono.framework
+      - sudo rm -r /Library/Frameworks/Mono.framework
     artifacts:
       - path: build/reaper_reapack*.dylib