reproducible debian: build: declare and define the variables separately, to...

reproducible debian: build: declare and define the variables separately, to avoid masking the subshell return code

Here the `schroot --begin-session` call was failing, but the failure was
masking by the declaration being done at the same time, so the `set -e`
was just wasted.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>