prepare-source: Move go fmt to last step

When rebasing, the generated code ends up with conflict markers in it
and go fmt quits with an error. This change regenerates the code first
so that fmt is happy with it.

tools/prepare-source.sh

@@ -14,8 +14,8 @@ $GO_BINARY download
 $GO_BINARY mod tidy
 $GO_BINARY mod vendor
 
-# ... the code is formatted correctly, ...
-$GO_BINARY fmt ./...
-
 # ... and all code has been regenerated from its sources.
 $GO_BINARY generate ./...
+
+# ... the code is formatted correctly, ...
+$GO_BINARY fmt ./...