The nix actions cache currently leads to repeated "no space left on
devices" errors for jobs in CI.
The prepopulate job is useless without the nix actions cache, so it
will go away at the same time.
* ci: try nix-community/cache-nix-action
* ci: add cache-id param to setup-nix action
* ci: tidy up cache keys for non-nix jobs
* ci: merge-nix-caches-linux job
* ci: merge caches other way around
* ci: reduce number of caches
Should prevent disk overflow
* ci: change cache id prefix for merge
* ci: comment out cache merging job
* ci: revert cache id prefix
* ci: setup-nix: use latest cache-nix-action
Among others, makes action logs look more tidy (see https://github.com/nix-community/cache-nix-action/commit/17d19d3d8be918757589635bc1a6830be0b129d2)
* ci: use test-pg cache key for loadtest
