static checks (with no approval required) running out of disk space in GH-hosted runners

[isegall-da]

See e.g. https://github.com/hyperledger-labs/splice/actions/runs/20715879130/job/59466819233?pr=3426

This is running on the free github-hosted runners. Seems like our nix setup has grown beyond the disk size provided on them. Fixing that would probably require a version of the nix config that includes only a subset required for the static tests.

But we should also check if we don't use nix on a GH-hosted runner anywhere else.

[krzysztofczyz-da]

Notes:

• nix store untared currently takes 13.2G of space
• the cache tarball currently takes 5.3G of space
• space for GH-hosted runner is 14GB - link

Ideas:

• use FUSE to GCS to not download the tarball for the testing - not viable, but potentially useful for the future
• shrink the package no. & update CI config
  • create a new nix flake
  • integrate into current nix flake

Currently pursuing the second idea.