add HC_TARGET_PREFIX from holonix and rename

[zippy]

PR summary

Holonix used to set the directory where HC incremental compilation went to nix-holochain that value should really be set in this repo. Also renamed it to something more reasonable: .holochain-compile-cache

testing/benchmarking notes

( if any manual testing or benchmarking was/should be done, add notes and/or screenshots here )

followups

( any new tickets/concerns that were discovered or created during this work but aren't in scope for review here )

changelog

Please check one of the following, relating to the CHANGELOG-UNRELEASED.md

• this is a code change that effects some consumer (e.g. zome developers) of holochain core so it is added to the CHANGELOG-UNRELEASED.md (linked above), with the format - summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)
• this is not a code change, or doesn't effect anyone outside holochain core development

[thedavidmeister]

@zippy this is super old, i forgot about it, FYI needs to be reviewed in context of #1973