Ensure generate-copyright is executed from the project root.

src/bootstrap/src/core/build_steps/run.rs

@@ -218,6 +218,9 @@ fn run(self, builder: &Builder<'_>) -> Self::Output {
         cmd.env("DEST_LIBSTD", &dest_libstd);
         cmd.env("OUT_DIR", &builder.out);
         cmd.env("CARGO", &builder.initial_cargo);
+        // it is important that generate-copyright runs from the root of the
+        // source tree, because it uses relative paths
+        cmd.current_dir(&builder.src);
         cmd.run(builder);
 
         vec![dest, dest_libstd]

src/tools/generate-copyright/src/main.rs

@@ -8,7 +8,10 @@
 
 /// The entry point to the binary.
 ///
-/// You should probably let `bootstrap` execute this program instead of running it directly.
+/// You should probably let `bootstrap` execute this program instead of running
+/// it directly. It assumes that the current working directory is the root of a
+/// Rust git repository checkout, and constructs a bunch of relative paths based
+/// on that assumption.
 ///
 /// Run `x.py run generate-copyright`
 fn main() -> Result<(), Error> {