ci: Always store cache (and use cache for build-repo)

It seems that despite the documentation mentioning nothing about it, you
can always save a cache even if a job fails. Which is good for us as we
are caching the nix stuff, which often works when the whole job fails.

See: https://gitlab.com/gitlab-org/gitlab/-/issues/18969

The other option here (which introduces some more complexity) is to
separate the building of the tools into a separate job which then each
runner will download and unpack.

.gitlab-ci.yml

@@ -169,6 +169,7 @@ test-9.4:
 
   cache:
     key: build-HEAD
+    when: always
     paths:
       - store.nar
 
@@ -218,6 +219,12 @@ test-9.4:
 update-repo:
   stage: update-repo
 
+  cache:
+    key: build-HEAD
+    when: always
+    paths:
+      - store.nar
+
   tags:
     - x86_64-linux
     - head.hackage