Fix non english locale

```ruby
qpassword = Yast::String.Quote password
result = Yast::SCR.Execute(YAST_BASH_PATH,
  "echo '#{qpassword}\n#{qpassword}\n' | grub2-mkpasswd-pbkdf2"
)
```
This works because:
1. `Yast::SCR.Execute(YAST_BASH_PATH, "whatever '#{Quote arg1}' '#{Quote arg2}'")` is the tested way for the system agent quoting.
2. We let Ruby handle the LF characters.

A temporary file would work too:
```ruby
SCR.Write(.target.string, filename, "#{password}\n#{password}\n")
SCR.Execute(.target.bash_output, "grub2-mkpasswd-pbkdf2 < #{filename}")
```