Add downstream project build testing

ci/buildkite-pipeline.sh

@@ -175,6 +175,30 @@ EOF
       "Stable-perf skipped as no relevant files were modified"
   fi
 
+  # Downstream backwards compatibility
+  if affects \
+             .rs$ \
+             Cargo.lock$ \
+             Cargo.toml$ \
+             ^ci/rust-version.sh \
+             ^ci/test-stable-perf.sh \
+             ^ci/test-stable.sh \
+             ^ci/test-local-cluster.sh \
+             ^core/build.rs \
+             ^fetch-perf-libs.sh \
+             ^programs/ \
+             ^sdk/ \
+             ^scripts/build-downstream-projects.sh \
+      ; then
+    cat >> "$output_file" <<"EOF"
+  - command: "scripts/build-downstream-projects.sh"
+    name: "downstream-projects"
+    timeout_in_minutes: 30
+EOF
+  else
+    annotate --style info \
+      "downstream-projects skipped as no relevant files were modified"
+  fi
   # Benches...
   if affects \
              .rs$ \