Improve Jenkins tests

Dockerfile.jenkins

@@ -1,4 +1,5 @@
 FROM schemers/chibi:head
 RUN apt-get update && apt-get install -y \
     gcc ca-certificates libffi-dev docker.io make
-RUN snow-chibi install --impls=chibi --always-yes retropikzel.test-r7rs
+ARG timestamp=0
+RUN snow-chibi install --impls=chibi --always-yes retropikzel.test-r7rs && echo "${timestamp}"

Jenkinsfile

@@ -4,6 +4,7 @@ pipeline {
             label 'docker-x86_64'
             filename 'Dockerfile.jenkins'
             args '--user=root --privileged -v /var/run/docker.sock:/var/run/docker.sock'
+            additionalBuildArgs "--build-arg timestamp=${currentBuild.startTimeInMillis}"
             reuseNode true
         }
     }