Make source match crate contents.

Restore env!("CARGO_PKG_VERSION") since Soong handles it.

Bug: http://b/339424309
Test: treehugger
Change-Id: I595e175cd542c787c3905ff1b5e3833734e158d6
diff --git a/2.27.1/src/lib.rs b/2.27.1/src/lib.rs
index 826e18b..1756a44 100644
--- a/2.27.1/src/lib.rs
+++ b/2.27.1/src/lib.rs
@@ -254,7 +254,7 @@
     {
         let mut w = CodeWriter::new(&mut v);
 
-        w.write_generated_by("rust-protobuf", "2.27.1");
+        w.write_generated_by("rust-protobuf", env!("CARGO_PKG_VERSION"));
         w.write_line(&format!("//! Generated file from `{}`", file.get_name()));
         if customize.inside_protobuf != Some(true) {
             w.write_line("");