Ignore failure when trying to delete lock file.

Deleting the lock file can fail when another process is currently trying to acquire it (-> race condition).
It is safe to ignore this error since the other process will eventually acquire the lock and again try to delete the lock file.

lib/vagrant/environment.rb

@@ -484,7 +484,11 @@ module Vagrant
       if name != "dotlock"
         lock("dotlock", retry: true) do
           f.close
-          File.delete(lock_path)
+          begin
+            File.delete(lock_path)
+          rescue
+            @logger.debug("Failed to delete lock file #{lock_path} - some other thread might be trying to acquire it -> ignoring this error")
+          end
         end
       end