ci-config: best-effort push CI intermediate build results to attic

If the builder is set up with an attic configuration (as is the case
since today on the Lix Buildkite builders), every Buildkite build step
attempts to push all their output store paths to attic so they can be
reused in later builds and/or in further steps being scheduled on
different builders.

Change-Id: Id102be66ede70c46ca8c34e1c929977edb4d5f57

calculate-buildkite-steps.py

@@ -116,7 +116,11 @@ try:
             'group': group,
             'steps': [
                 {
-                    'command': f'nix build --no-link -L ".#hydraJobs.{job.name}"',
+                    'command': textwrap.dedent(f'''
+                        set -ueo pipefail
+                        resultPath=$(nix build --print-out-paths --no-link -L ".#hydraJobs.{job.name}")
+                        attic push afnix-ci $$resultPath || true
+                    '''),
                     'label': ".".join(job.path),
                     'key': job.drv_hash(),
                     'depends_on': [ d.drv_hash() for d in deps[job] ],