gitlab-ci: Fix sysroot index URL generation

Commit 943e1ce5 "gitlab-ci: No need to source the build-env explicitly" has been a bit too eager in moving variables from the shell script to the YAML and as a result we ended up generating a broken URL for the sysroot index.

The issue lies in sysrooturl as is defined to use sysrootname which in turns uses other variables, but since they are defined at the same level those variables are not expanded when expanding sysrootname in sysrooturl, yielding an URL like:

https://images.apertis.org/daily/v2021pre/20201201.1206/armhf/sysroot/sysroot-${osname}-${release}-${architecture}-${PIPELINE_VERSION}.tar.gz

Move back the definition of sysrooturl to the shell script so variables are expanded correctly.