feat(ops/pipelines): Fetch parent target map for pipeline generation

Change-Id: I1c7d48fc0974549d67146a15f79ddb0b6ddfe805
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4947
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/pipelines/static-pipeline.yaml

@@ -15,7 +15,17 @@ steps:
           buildkite-agent annotate
       fi
 
-      nix-build -A ops.pipelines.depot -o pipeline --show-trace
+      # Attempt to fetch a target map from a parent commit on canon,
+      # except on builds of canon itself.
+      [ "${BUILDKITE_BRANCH}" != "refs/heads/canon" ] && \
+        ops/pipelines/fetch-parent-targets.sh
+
+      PIPELINE_ARGS=""
+      if [[ -f ./parent-target-map.json ]]; then
+        PIPELINE_ARGS="--arg parentTargetMap ./parent-target-map.json"
+      fi
+
+      nix-build -A ops.pipelines.depot -o pipeline --show-trace $$PIPELINE_ARGS
 
       # Steps need to be uploaded in reverse order because pipeline
       # upload prepends instead of appending.