[Jenkins] Reordered pipeline for better concurrency.

Things to be built on master only are now executed in a stages
post() part. This allows for more parallelism of the sub-jobs.

More changes:

- Removed setting of option OGS_CPU_ARCHITECTURE on eve frontend
  builds
- Failed header compilation in stage 'Check headers' marks the
  build as unstable
- Tests-Large now runs on singularity1 or envinf1