Support snapshot-location-base

[hasufell]

https://github.com/commercialhaskell/stack/blob/baa894b12b5b13330e340561cdd1040819498656/doc/yaml_configuration.md#snapshot-location-base

---

For example:

snapshot-location-base: https://example.com/snapshots/location/

has the following effect:

• lts-X.Y expands to https://example.com/snapshots/location/lts/X/Y.yaml
• nightly-YYYY-MM-DD expands to https://example.com/snapshots/location/nightly/YYYY/M/D.yaml