Add a realisations disk cache

Similar to the nar-info disk cache (and using the same db).
This makes rebuilds muuch faster.

- This works regardless of the ca-derivations experimental feature.
  I could modify the logic to not touch the db if the flag isn’t there,
  but given that this is a trash-able local cache, it doesn’t seem to be
  really worth it.
- We could unify the `NARs` and `Realisation` tables to only have one
  generic kv table. This is left as an exercise to the reader.
- I didn’t update the cache db version number as the new schema just
  adds a new table to the previous one, so the db will be transparently
  migrated and is backwards-compatible.

Fix #4746

tests/ca/substitute.sh

@@ -45,3 +45,16 @@ if [[ -z "$(ls "$REMOTE_STORE_DIR/realisations")" ]]; then
     echo "Realisations not rebuilt"
     exit 1
 fi
+
+# Test the local realisation disk cache
+buildDrvs --post-build-hook ../push-to-store.sh
+clearStore
+# Add the realisations of rootCA to the cachecache
+clearCacheCache
+export _NIX_FORCE_HTTP=1
+buildDrvs --substitute --substituters $REMOTE_STORE --no-require-sigs -j0
+# Try rebuilding, but remove the realisations from the remote cache to force
+# using the cachecache
+clearStore
+rm $REMOTE_STORE_DIR/realisations/*
+buildDrvs --substitute --substituters $REMOTE_STORE --no-require-sigs -j0