build-systsem: move back to Xcode 11

It appears that Xcode 12 applies some rather self defeating logic when picking
build architectures in release builds for the simulator. It adds aarch64 by
default and I can't find a way to turn that off from the command line. At the
same time, you can't link against the simulator if you have build with aarch64
as the aarch64 simulator doesn't exist, yet.

Since I couldn't get any of the claimed workarounds to work, I'm forcing Xcode
11 to be used in the Action.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.github/workflows/ios.yml

@@ -9,8 +9,10 @@ on:
 
 jobs:
   mobileBuild:
-    runs-on: macOS-latest
+    runs-on: macOS-10.15
     steps:
+    - name: switch to Xcode 11
+      run: sudo xcode-select -s "/Applications/Xcode_11.7.app"
     - name: checkout sources
       uses: actions/checkout@v1
     - name: setup Homebrew