The colon variants of withenv env update operators do not adding leading/trailing separators

[gridbugs]

The := and =: operators inside the withenv action do not add leading/trailing separators when operating on unset variables. These operators are documented in opam's manual as:

+= or := prepend. They differ when the variable is unset of empty, where := adds a trailing separator. =+ or =: append. They differ when the variable is unset of empty, where =: adds a leading separator.

Repro: https://github.com/ocaml/dune/pull/10441