buildkite: force UTF-8 encoding in custom pipeline

This prevents Unicode encoding errors when printing
to stdout from the benchmark integ test.

Signed-off-by: Alexandra Iordache <aghecen@amazon.com>

Author: Alexandra Iordache

.buildkite/bench_pipeline.yml

@@ -12,6 +12,8 @@ steps:
       - docker#v3.0.1:
           image: "rustvmm/dev:v6"
           always-pull: true
+          environment:
+            - "PYTHONIOENCODING=utf-8"
           propagate-environment: true
 
   - label: "bench-aarch64"
@@ -25,4 +27,6 @@ steps:
       - docker#v3.0.1:
           image: "rustvmm/dev:v6"
           always-pull: true
+          environment:
+            - "PYTHONIOENCODING=utf-8"
           propagate-environment: true