core: fix file URLs from cygwin on Windows [GH-5747]

CHANGELOG.md

@@ -72,6 +72,7 @@ BUG FIXES:
       retry from scratch [GH-4479]
   - core: line numbers show properly in Vagrantfile syntax errors
       on Windows [GH-6445]
+  - commands/box: add command with `~` paths on Windows works [GH-5747]
   - commands/box: the update command supports CA settings [GH-4473]
   - commands/box: removing all versions and providers of a box will properly
       clean all directories in `~/.vagrant.d/boxes` [GH-3570]

lib/vagrant/action/builtin/box_add.rb

@@ -381,7 +381,8 @@ module Vagrant
             @logger.info("URL is a file or protocol not found and assuming file.")
             file_path = File.expand_path(url)
             file_path = Util::Platform.cygwin_windows_path(file_path)
-            url = "file:#{file_path}"
+            file_path = file_path.gsub("\\", "/")
+            url = "file://#{file_path}"
           end
 
           # If the temporary path exists, verify it is not too old. If its