Some modifications to improve the benchmark pipeline

• Use time-machine to deploy a fixed environment: generate a channels.locked.scm file and deploy from there
• Add missing Guix-HPC channel from channels.scm as some packages needed by the CI depend on it (jube-with-yaml for example). Everything was working because the pipeline was implicitly using the Guix environment on plafrim which is configured with Guix HPC.
• Don't use expensive guix_env.sh script when generating pages