Copy `mold` to the top source directory

test/gentoo-test.sh depends on the presence of the file.

.gitignore

@@ -7,3 +7,4 @@ core
 gentoo
 /mold-*.tar.gz
 /build*
+/mold

dist.sh

@@ -166,6 +166,7 @@ cmake --install . --prefix $dest --strip
 find $dest -print | xargs touch --no-dereference --date='$timestamp'
 find $dest -print | sort | tar -cf - --no-recursion --files-from=- | gzip -9nc > /mold/$dest.tar.gz
 chown $(id -u):$(id -g) /mold/$dest.tar.gz
+cp mold /mold
 "
 
 which sha256sum > /dev/null && sha256sum $dest.tar.gz