os: replace PATH_SEP with std.fs.path.delimiter

This standard library symbol is equivalent.

src/termio/Exec.zig

@@ -780,7 +780,7 @@ const Subprocess = struct {
             // then we just set it to the directory of the binary.
             if (env.get("PATH")) |path| {
                 // Verify that our path doesn't already contain this entry
-                var it = std.mem.tokenizeScalar(u8, path, internal_os.PATH_SEP[0]);
+                var it = std.mem.tokenizeScalar(u8, path, std.fs.path.delimiter);
                 while (it.next()) |entry| {
                     if (std.mem.eql(u8, entry, exe_dir)) break :ghostty_path;
                 }