Do not fail if SKIP_DCH is unset; use 'true' for checking for SKIP_DCH; inform user about it
Do not fail if SKIP_DCH is unset; use 'true' for checking for SKIP_DCH; inform user about it

The script runs under 'set -u' so we have to use "${SKIP_DCH:-}"
to make sure the script doesn't fail if SKIP_DCH is *not* set
(being the default usually).

We already check for 'true' in different settings, so let's keep
it homogeneous and use 'true' instead of '1'.

Also inform the user about when SKIP_DCH is being used to make
debugging easier.

Related to previous commit (201ec1e378302391f360ff84e04de6efd124482f).

hashar/jenkins-debian-glue,kartikm/jenkins-debian-glue,n054/jenkins-debian-glue,n054/jenkins-debian-glue,kartikm/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,vorlock/jenkins-debian-glue,mika/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,jbfavre/jenkins-debian-glue,zeha/jenkins-debian-glue,zeha/jenkins-debian-glue,tanglu-org/jenkins-tanglu-buildkit,webtrekk-kane/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,tonin/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,hashar/jenkins-debian-glue,tanglu-org/jenkins-tanglu-buildkit,cclausen/jenkins-debian-glue,jbfavre/jenkins-debian-glue,mika/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,cclausen/jenkins-debian-glue,vorlock/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,hetznerZA/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,tonin/jenkins-debian-glue,tanglu-org/jenkins-tanglu-buildkit