ci: add KAS_REPO_REF_DIR in persistent storage

This variable can be used to specify where to clone all repos used by
kas. By setting up a persistent storage location, we can keep a copy
of all git trees locally, and only fetch new objects. So kas will
first clone/fetch objects in KAS_REPO_REF_DIR location, and then will
use them as a source when checking out the repo for the build.

The documentation says that:

  Multiple instances of kas can simultaneously work on the same
  directory, as long as the underlying filesystem is POSIX compatible.

So it is designed to be safe in CI environments such as ours.

New trees are cloned the first time they are encountered, and only new
objects are fetched otherwise.

Note: we cannot concatenate global env variables, hence a little bit
of duplication between KAS_REPO_REF_DIR and CACHE_DIR.

Signed-off-by: Nicolas Dechesne <[email protected]>

.github/workflows/build-yocto.yml

@@ -9,6 +9,7 @@ on:
 
 env:
   CACHE_DIR: /srv/gh-runners/quic-yocto
+  KAS_REPO_REF_DIR: /srv/gh-runners/quic-yocto/kas-mirrors
 
 jobs:
   kas-lock: