Fix path separator for Windows in git hook generation

The gradleRootDirPrefix was using platform-specific path separators
(backslash on Windows), but git hook scripts always require forward
slashes. This caused tests to fail on Windows.

Changed to always use forward slashes by replacing File.separator
with "/" when constructing gradleRootDirPrefix.

Fixes test failures on windows-latest CI runners.

Author: hey-cube

plugin/src/main/kotlin/org/jlleitschuh/gradle/ktlint/GitHook.kt

@@ -190,7 +190,7 @@ open class KtlintInstallGitHookTask @Inject constructor(
             gitHookFile.createNewFile()
             gitHookFile.setExecutable(true)
         }
-        val gradleRootDirPrefix = File(rootDirectory.get()).relativeTo(repo.workTree).path
+        val gradleRootDirPrefix = File(rootDirectory.get()).relativeTo(repo.workTree).path.replace(File.separator, "/")
 
         if (gitHookFile.length() == 0L) {
             gitHookFile.writeText(