Receipt files should be named deterministicly

Currently, experiment receipt files are named `--.receipt`.

I see little value in having such an elaborate naming scheme and it makes automation scenarios slightly more challenging.

Instead, the receipt files should be named consistently across all experiments and build tools, like `experiment.receipt`.

This would make finding the receipt file much easier. For example:

```
cat .data/01-validate-incremental-building/latest/experiment.receipt
```

Would always print the receipt for the most recent invocation of experiment 1.

If #619 were implemented, then:

```
cat .data/latest/experiment.receipt
```

Would always get you the receipt of the most recent invocation across all experiments.