Update charlievieth/fastwalk to use forward-slashes on WSL and MSYS (#3907)

This commit changes FZF to enforce that all paths are joined with
forward-slashes when running on WSL or MSYS
even when the FZF binary was compiled for Windows.

Update: github.com/charlievieth/fastwalk
Fixes:  https://github.com/junegunn/fzf/issues/3859

---------

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

go.mod

@@ -1,7 +1,7 @@
 module github.com/junegunn/fzf
 
 require (
-	github.com/charlievieth/fastwalk v1.0.4
+	github.com/charlievieth/fastwalk v1.0.7-0.20240703190418-87029d931815
 	github.com/gdamore/tcell/v2 v2.7.4
 	github.com/mattn/go-isatty v0.0.20
 	github.com/mattn/go-shellwords v1.0.12