RunCommand: provide build inputs information

[Taneb]

Currently, there is no way to go from the JSON file RunCommand provides to the build inputs, which I believe are stored with the build in Hydra's database.

I want this to be able to log a git commit with some statistics in an external service on each build, and also separately to run a program kept in the same git repository that is not suitable to do as part of a nix build, making sure the program is kept in sync.

It's currently possible to fake this by scraping the web interface, but that is an unsatisfying way to achieve this.

[grahamc]

I took a look at this, but it isn't so clear cut if that would be useful in a principled way. What I mean is that the RunCommand plugin only executes once, while that same output might be part of many different evaluations, with many different inputs. Perhaps it is useful still, but if you want to create a map of all the inputs used to produce an output -- it may not be the complete log you're expecting -- just the first time. What do you think?