address comments

.ci/compile.sh

@@ -156,6 +156,18 @@ function ccachestatsverbose() {
 
 # Compile
 if [[ $RUNNER_OS == macOS ]]; then
+  # QTDIR is needed for macOS since we actually only use the cached thin Qt binaries instead of the install-qt-action,
+  # which sets a few environment variables
+  if QTDIR=$(find "$GITHUB_WORKSPACE/Qt" -depth -maxdepth 2 -name macos -type d -print -quit); then
+    echo "found QTDIR at $QTDIR"
+  else
+    echo "could not find QTDIR!"
+    exit 2
+  fi
+  # the qtdir is located at Qt/[qtversion]/macos
+  # we use find to get the first subfolder with the name "macos"
+  # this works independent of the qt version as there should be only one version installed on the runner at a time
+  export QTDIR
 
   if [[ $TARGET_MACOS_VERSION ]]; then
     # CMAKE_OSX_DEPLOYMENT_TARGET is a vanilla cmake flag needed to compile to target macOS version

.ci/thin_macos_qtlib.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+# The macos binaries from aqt are fat (universal), so we thin them to the target architecture to reduce the size of
+# the packages and caches using lipo.
+
+# This script is meant to be used by the ci enironment on macos runners only.
+# It uses the runner's GITHUB_WORKSPACE env variable.
+arch=$(uname -m)
+nproc=$(sysctl -n hw.ncpu)
+
+function thin() {
+  local libfile=$0
+  if file -b --mime-type "$libfile" | grep -q "application/x-mach-binary"; then
+    echo "Processing $libfile"
+    lipo "$libfile" -thin "$arch" -output "$libfile"
+  fi
+  return 0
+}
+export -f thin  # export to allow use in xargs
+
+echo "::group::Thinning Qt libraries to $arch using $nproc cores"
+find "$GITHUB_WORKSPACE/Qt" -type f -print0 | xargs -0 -n1 -P"$nproc" -I{} bash -c "thin '{}'"
+echo "::endgroup::"