This should significantly speedup building the pinecone and yggdrasil
demos, as they now reuse the new `dendrite:buildcache`, which already
builds all binaries.
### Pull Request Checklist
* [x] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately
No tests were added due to it being a small documentation addition
Signed-off-by: `Cat Catry <denperidge@gmail.com>`
