Skip valgrind tests on MacOS github runners

spec/propane_spec.rb

@@ -95,6 +95,9 @@ EOF
     File.binwrite("spec/run/.stdout", stdout)
     stderr.sub!(/^.*modules passed unittests\n/, "")
     results = Results.new(stdout, stderr, status)
+    # Valgrind is only reliably available on Linux, so limit the leak check to
+    # Linux platforms.
+    if RUBY_PLATFORM =~ /linux/
       stdout, stderr, status = Open3.capture3("valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --verbose spec/run/testparser")
       vgout = stdout + stderr
       File.binwrite("spec/run/.vgout", vgout)
@@ -104,6 +107,7 @@ EOF
           raise "Valgrind detected memory leak"
         end
       end
+    end
     results
   end