Update buildkite/plugin-tester to v3.0.1

There was a big bug in v3.0.0, but one that the test suite in this
repository didn't trigger. We'll go ahead an update to remove the bug
and remove a potential source of confusion in the future.

- https://github.com/buildkite-plugins/buildkite-plugin-tester/releases/tag/v3.0.1
- buildkite-plugins/bats-mock#8

Signed-off-by: Christopher Maier <[email protected]>

Author: christophermaier

docker-compose.yml

@@ -8,7 +8,7 @@ x-common-variables:
 
 services:
   plugin-tester:
-    image: buildkite/plugin-tester:v3.0.0
+    image: buildkite/plugin-tester:v3.0.1
     volumes:
       - *read-only-plugin