windows: skip expandHomeUnix test on Windows

expandHomeUnix is a Unix-internal function that is never called on
Windows. The public expandHome function returns the path unchanged
on Windows since ~/ is not a standard Windows idiom. The test calls
expandHomeUnix directly, which invokes home() and expects Unix-style
forward-slash separators.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/os/homedir.zig

@@ -149,6 +149,8 @@ fn expandHomeUnix(path: []const u8, buf: []u8) ExpandError![]const u8 {
 }
 
 test "expandHomeUnix" {
+    if (builtin.os.tag == .windows) return error.SkipZigTest;
+
     const testing = std.testing;
     const allocator = testing.allocator;
     var buf: [std.fs.max_path_bytes]u8 = undefined;