Changes

jobs/osmo-gsm-tester-build: fix Missing parameter

Use OSMO_GSM_TESTER_BRANCH as shell variable ('$...'), not as jenkins
job builder parameter (${...}). This is how we do it in other jobs as
well, e.g. jobs/osmocom-obs.yml.

Apparently the workaround in the file happened to work earlier, but it
does not work anymore with Jenkins Job Builder version 6.3.0. Remove it.

Fix for:
  osmo-gsm-tester-builder.yml:119:15: While formatting string '${OSMO_GSM_TESTER_BRANCH}': Missing parameter: 'OSMO_GSM_TESTER_BRANCH'
                - ${OSMO_GSM_TESTER_BRANCH}
                  ^

Change-Id: I3b6b16964a4d8a51fd3ecc0d670805182a8c6586

libosmo-sccp -> libosmo-sigtran

Adjust to:
* libosmo-sccp.git has been renamed to libosmo-sigtran.git
* libosmo-sccp static libraries have been split into
  libosmo-sccp-legacy.git

build-old-tags-against-master: libosmo-sccp is intentionally kept in
  REPOS_TAGS, so old libosmo-sccp versions are built against current
  master of Osmocom libraries.

Related: OS#6239
Change-Id: I509b1cc2a5260c82ef32e9543513110949311188