Introduce separate variables to set RELEASE_NAME and RELEASE_VERSION

The expressions to calculate RELEASE_NAME and RELEASE_VERSION cannot be
stored within these variables themseselves. If set on the trigger level,
they would override the calculated values in build-version.env then,
as trigger variables always have the highest precedence.

Use separate variables RELEASE_VERSION_EXPRESSION and
RELEASE_NAME_EXPRESSION to define how RELEASE_VERSION and RELEASE_NAME
are calculated instead.