Add param `lock` to `update()`

[Salim B]

I wondered whether the recently introduced param lock in install() could also be added to update(), allowing the renv.lock lockfile to be updated automatically after successful update of the requested packages. It would be be a convenient feature for users of renv's "explicit" snapshot type.

[Kevin Ushey]

This was an easy lift, given that renv::update() calls renv::install() in the end -- I've just added this.