There is a --with-source option. So you could actually specify the download location to the 2.10 version https://www.gnu.org/software/guix/manual/en/html_node/Package-Transformation-Options.html
Also have you tried building it by specifying "biber@2.10" instead of just "biber"? Sorry I can't say much about the environment command, other than it looks correct to me.
