Closes#987
The patch adds a flag `--no-instantiate` which only performs evaluation
without instantiating any derivations. Hence, GC root creation is also
skipped. To achieve that, Lix is also put in read-only mode and all
operations that require reading a derivation (e.g. constituents or
listing input derivations) are disabled fallback values are set.
This is a port of an upstream PR[1]. Given the divergence of the
codebases (different restructurings on both ends, no more CA derivations)
I decided to redo large portions from scratch instead of
cherry-picking the patches. Hence, the authorship.
Additionally the clean up of casts down to a local store are removed or
guarded behind an if, as done in the upstream PR.
[1] https://github.com/nix-community/nix-eval-jobs/pull/379
Co-authored-by: Jörg Thalheim <joerg@thalheim.io>
Change-Id: Ib84f44e7799bc5577fd2ee98912458f16ebeab81
