Jenkinsfile: Fix arch mismatches in sysroot index file generation

Fix indentation and scope in the buildSysroot(), which may actually fix the weird mismatching sysroot index file generation error.

See https://phabricator.apertis.org/T6317

In particular, the issue led to the generation of sysroot index files where the architecture of the contents didn't match the expected one.

For instance, see the arm64 job below, pointing to armhf artifacts: https://jenkins.apertis.org/job/apertis-v2019/job/images/job/debos-image-build/55/execution/node/474/log/

My suspect is that since sysrootname and sysrooturl where generated outside of the scope of the stage(), they got overridden before the lazy evaluation of the stage kicked in: in the case above, the arm64 job probably ran first, but before its stage() got evaluated the loop moved to armhf and updated ``sysrootnameandsysrooturl`, so that when the `arm64` stage actually got evaluated it picked up the new `armhf` values.