CI: Mention issue preventing check to work on Mac OS

Ref micropython/micropython#5500

Author: jonnor

.github/workflows/build.yaml

@@ -210,6 +210,7 @@ jobs:
         mv ./dist/ports/unix/micropython ./dist/ports/macos/micropython
     - name: Build .mpy modules and run tests Unix/x64
       # natmod build with linking seems broken on Mac OS, exception in mpy_ld.py, ELFError, magic number does not match
+      # https://github.com/micropython/micropython/issues/5500
       run: echo SKIP make check ARCH=x64 V=1
 
     - name: Archive dist artifacts