Issue 5100: Prevent race condition in font export directory making

Issue 5100: Prevent race condition in font export directory making

When a font export directory is necessary,
LilyPond tested the existence of the directory,
and if the directory did not exist, LilyPond made the directory.
However, LilyPond raised the error that the directory already exists
if another process made the directory between the testing and the making.

This commit prevents such race condition.
By deleting the existence test,
LilyPond always tries to make the directory regardless of existence.
Then suppress the error that the directory already exists.