conda packages generated by the robotology-superbuild should prepend values to existing environment variables, not append

[traversaro]

For a long time, in all activation scripts I always "appended" values to existing environment variables. My reasoning was to be "kind" with the environment. However, in the case of conda packages, this does not make sense: if there is something defined at the system level, the latest activated environment should have the precedence, not the first.

cc @Nicogene @xEnVrE

[Nicogene]

cc @martinaxgloria