Fixes #11218: Refresh apk cache when adding rsync

This ensures that rsync can be installed on an Alpine Linux system where
the apk cache may not be current.

plugins/guests/alpine/cap/rsync.rb

@@ -8,7 +8,7 @@ module VagrantPlugins
 
         def self.rsync_install(machine)
           machine.communicate.tap do |comm|
-            comm.sudo('apk add rsync')
+            comm.sudo('apk add --update-cache rsync')
           end
         end
       end

test/unit/plugins/guests/alpine/cap/rsync_test.rb

@@ -16,9 +16,9 @@ describe 'VagrantPlugins::GuestAlpine::Cap::RSync' do
         VagrantPlugins::GuestAlpine::Plugin.components.guest_capabilities[:alpine].get(:rsync_install)
     end
 
-    it 'should install rsync' do
+    it 'should install rsync with --update-cache flag' do
         # communicator.should_receive(:sudo).with('apk add rsync')
-        expect(communicator).to receive(:sudo).with('apk add rsync')
+        expect(communicator).to receive(:sudo).with('apk add --update-cache rsync')
         allow_message_expectations_on_nil
         described_class.rsync_install(machine)
     end