Windows build is failing, maybe fileio_spec.lua:94 is sending nil ?

I'd be very grateful if someone with a Windows machine would be able to correct this test.

I don't have windows build env available.
But I think the problem is read_file('Xtest_backupdir' .. sep .. backup_file_name) return nil so trim complain.
Can someone with windows build can build this PR, edit a file after setting set backupdir=whatever// and post the resulting backup file name here ?

one can debug this by adding temporary changes and observing the result on CI: 1d5c3e6

Result:

test/functional\core\fileio_spec.lua:110: Expected objects to be the same.
Passed in:
(table: 0x0cdaf498) {
  [1] = 'C:\projects\neovim'
  [2] = 'C:%projects%neovim%Xtest_startup_file1~'
  [3] = 'Xtest_backupdir\C%%projects%neovim%Xtest_startup_file1~' }
Expected:
(string) 'x'
stack traceback:

• So it looks like the : needs to be replaced with %.
  • I pushed a commit that does so.
• Also removed the sep stuff, because read_file works with / separators.

Thanks @expipiplus1

Followup

• Setting backupdir with dirs that don't exist and linked files  #11349 is somewhat related, could use some help there if you're interested.
• This was the thing blocking us from auto-creating 'backupdir', similar to how we auto-create the directories for 'directory' and 'undodir'. We could move forward with that now.

Thanks!