cea95f96

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

share nextest test compilation across CI groups via cargo nextest archive

Each of the five gammaloop-nextest-* derivations previously recompiled
its packages' test binaries on top of the shared cargoArtifacts blob,
duplicating any compile work for crates that appear in multiple groups.
Build a single workspace-wide nextest archive once with
craneLib.mkCargoDerivation, expose it as a flake package so NixCI
schedules it on its own worker, and have each group derivation run
prebuilt tests with --archive-file --workspace-remap . instead of
invoking cargo build.