(build) update mono to v 6.4.0

Author: arturcic

build/stages/common-steps.yml

@@ -24,10 +24,8 @@ steps:
   displayName: 'Use mono'
   inputs:
     script: |
-      brew install mono
-      brew link --overwrite mono
-      SYMLINK=6.0.0.313
-      MONOPREFIX=/usr/local/Cellar/mono/$SYMLINK
+      SYMLINK=6.4.0
+      MONOPREFIX=/Library/Frameworks/Mono.framework/Versions/$SYMLINK
       echo "##vso[task.setvariable variable=DYLD_FALLBACK_LIBRARY_PATH;]$MONOPREFIX/lib:/lib:/usr/lib:$DYLD_LIBRARY_FALLBACK_PATH"
       echo "##vso[task.setvariable variable=PKG_CONFIG_PATH;]$MONOPREFIX/lib/pkgconfig:$MONOPREFIX/share/pkgconfig:$PKG_CONFIG_PATH"
       echo "##vso[task.setvariable variable=PATH;]$MONOPREFIX/bin:$PATH"