nix-builder: cut down eval time by 31.5%

[danieldk]

I did some profiling of the build of ReLU (since it compiles with all backends) and found and fixed two hot spots:

• The pandoc package is used by a lot of packages to convert docs. However, pandoc is written in Haskell and evaluation of pandoc evaluates a large number of packages in the Haskell package set. Resolve this by overriding pandoc with a different derivation that packages precompiled pandoc binaries. This cuts out the Haskell dependencies, removing 9.3s from the 42.6s of eval time.
• We evaluate Cargo.lock using importCargoLock. This creates a lot of derivations (fetchurl for every crate). We prefer this over cargoHash, because it'd require updating the hash every time we make a change that touches Cargo.lock. However, we did this relatively expensive eval two times, once for kernel-builder and once for kernels. Avoid the additional evaluation by sharing the same thunk between both derivations. This removes 4.1s from the 42.6s evaluation time.

Overview of the total savings:

Step	Eval time	Saved	% of original	% of previous step
Original	42.6 s	—	—	—
+ pandoc from upstream binaries	33.3 s	9.3 s	21.8%	21.8%
+ shared importCargoLock	29.2 s	4.1 s	9.6%	12.3%
Total	29.2 s	13.4 s	31.5%	—

[sayakpaul]

Neato

[sayakpaul]

Cool