fix 'explain' test on windows

It's not clear why, but it appears that writing the file with the same content ends up
appearing like the file hasn't changed on Windows, at least on the GitHub CI.

Author: evmar

tests/e2e/basic.rs

@@ -194,7 +194,7 @@ fn explain() -> anyhow::Result<()> {
     let out = space.run_expect(&mut n2_command(vec!["out"]))?;
     assert_output_contains(&out, "up to date");
 
-    space.write("in", "")?;
+    space.write("in", "x")?;
     let out = space.run_expect(&mut n2_command(vec!["-d", "explain", "out"]))?;
     // The main "explain" log line:
     assert_output_contains(&out, "explain: build.ninja:6: manifest changed");