integrate with reproseed to improve reproducibility of the executions! #173

Came up in the scope of

• https://github.com/asmacdo/fs8-chain

Could be opt-in as we did for con/duct, as to instruct to invoke sourceing reproseed.sh before invoking singularity but then you might need to improve it to make it easier to figure out which env vars were set (or just compare before/after but that is trickier).

Likely instead of git submodule just use git subtree to vendorize a copy but likely automate it somehow (Makefile rule?) for future updates to happen