Guix manifest definitions

Guix manifests are a way to describe an environment with multiple packages and package transformations. Together with a guix channels file they allow for reproduction of a specific software environment. With this one can reproducibly build and package (and archive):

$ cd ogs
$ guix time-machine -C scripts/guix/channels.scm -- shell -C -m scripts/guix/manifest.scm
# now in an isolated shell (like a container)
$ ogs --version
...
$ exit
# Create an Apptainer container image (can be archived for later reuse):
$ guix time-machine -C scripts/guix/channels.scm -- pack -m scripts/guix/manifest.scm \
  -RR --format=squashfs
...
building /gnu/store/ciids67bnl3j523xb1yrlq46piqsahs3-ogs-serial-coreutils-bash-squashfs-pack.gz.squashfs.drv...
/gnu/store/w3rbxgx70pf50dss76if2vv0rxa6iix8-ogs-serial-coreutils-bash-squashfs-pack.gz.squashfs

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?