macOS: Set Protobuf_ROOT during provisioning rather than in each config

Decouples provisioning from build configurations.

Pick-to: 6.9 6.8
Change-Id: Ied2b60b360ef5ec5aab1dd4b2e6a0b81032d2938
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

coin/provisioning/common/unix/install_protobuf.sh

@@ -92,6 +92,8 @@ cmake "$targetDir" -G"Ninja Multi-Config" \
 ninja all:all
 sudo env "PATH=$PATH" ninja install:all
 
+SetEnvVar "Protobuf_ROOT" "$installPrefix/lib/cmake/protobuf"
+
 # Refresh shared library cache if OS isn't macOS
 if uname -a |grep -qv "Darwin"; then
     sudo ldconfig