[CI] Downgrade macOS Github Action to macOS 10.15 (#9279)

Both BigSur and Monterey produce a broken `crengine.dylib`.

Only github actions are affected as local builds seem to work just fine: https://github.com/koreader/koreader/issues/8686#issuecomment-1166656492

.github/workflows/build.yml

@@ -7,8 +7,14 @@ permissions:
 
 jobs:
   macos_build:
-  
-    runs-on: macos-11
+
+    # macos-11 and macos-12 are broken at this time being.
+    # https://github.com/koreader/koreader/issues/8686,
+    # https://github.com/koreader/koreader/issues/8686#issuecomment-1172950236
+
+    # Please don't update to newer macOS version unless you can test that the new
+    # action produces working binaries.
+    runs-on: macos-10.15
 
     steps:
       - name: XCode version
@@ -29,7 +35,7 @@ jobs:
 
       - name: Building in progress…
         run: |
-          export MACOSX_DEPLOYMENT_TARGET=11;
+          export MACOSX_DEPLOYMENT_TARGET=10.15;
           export PATH="$(brew --prefix)/opt/gettext/bin:$(brew --prefix)/opt/gnu-getopt/bin:$(brew --prefix)/opt/bison/bin:$(brew --prefix)/opt/grep/libexec/gnubin:${PATH}";
           ./kodev release macos