fix(builtin.oldfiles): fix cwd_only for windows (#3096)

Remove backslash escaping in oldfiles picker, which was needed
before the code was changed to use substring comparison instead of
regex matching. (Commit b3ff5f33)

(cherry picked from commit 02a60a8396)

lua/telescope/builtin/__internal.lua

@@ -543,9 +543,9 @@ internal.oldfiles = function(opts)
     end
   end
 
-  if opts.cwd_only then
-    local cwd = vim.loop.cwd() .. utils.get_separator()
-    cwd = cwd:gsub([[\]], [[\\]])
+  if opts.cwd_only or opts.cwd then
+    local cwd = opts.cwd_only and vim.loop.cwd() or opts.cwd
+    cwd = cwd .. utils.get_separator()
     results = vim.tbl_filter(function(file)
       return buf_in_cwd(file, cwd)
     end, results)