Looks like our pinned test/flake.lock was being ignored and the latest
version of all inputs was being used. This should fix that oversight
(and give us better caching to boot)
* Define a new installation mode which symlinks identical artifacts
against a directory of previously generated ones
* This allows for linear space usage in the Nix store across many
chained derivations (as opposed to using a zstd compressed tarball
which uses quadratic space across many chained derivations)
* This new installation mode is the new default for all cargo based
builds. The previous behavior is still available by setting
`installCargoArtifactsMode = "use-zstd";` on a derivation
* `buildPackage` will continue to use zstd compressed tarballs while
building dependencies (unless either of `cargoArtifacts` or
`installCargoArtifactsMode` is defined, in which case they will be
honored)
