core: do not convert drive letters to UNC paths [GH-6598]

CHANGELOG.md

@@ -4,6 +4,8 @@ BUG FIXES:
 
   - core: Don't create ".bundle" directory in pwd [GH-6717]
   - core: Fix exception on installing VirtualBox [GH-6713]
+  - core: Do not convert standalone drive letters such as "D:" to
+      UNC paths [GH-6598]
   - commands/up: Smarter logic about what provider to install, avoiding
       situations where VirtualBox was installed over the correct provider [GH-6731]
   - guests/debian: Fix Docker install [GH-6722]

lib/vagrant/util/platform.rb

@@ -185,6 +185,12 @@ module Vagrant
         # @param [String] path Path to convert to UNC for Windows
         # @return [String]
         def windows_unc_path(path)
+          path = path.gsub("/", "\\")
+
+          # If the path is just a drive letter, then return that as-is
+          return path if path =~ /^[a-zA-Z]:\\?$/
+
+          # Convert to UNC path
           "\\\\?\\" + path.gsub("/", "\\")
         end