This will cache the output of `passwordCommand` per authentication
realm.
Context: the `credentials` key in `sbt` is a `TaskKey[Seq[Credentials]]`.
In `sbt`, tasks are evaluated on-demand and their output is not cached.
This particular key is referenced by all submodules in a project. When
the command is relatively expensive (e.g.: `pass show foo`), this
results in several seconds of delay when doing basic things like
`compile` or `test` which makes this unusable without some kind of
caching.
