ci: harden unit test step and junit post

Jenkinsfile

@@ -95,7 +95,7 @@ spec:
     stage('Unit tests') {
       steps {
         container('tester') {
-          sh '''
+          sh(script: '''
 set -euo pipefail
 python -m pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt
 mkdir -p build
@@ -114,7 +114,7 @@ spec:
 percent = summary.get("percent_covered")
 print(f"Coverage summary: {percent:.2f}%" if percent is not None else "Coverage summary unavailable")
 PY
-          '''
+'''.stripIndent())
         }
       }
     }
@@ -204,7 +204,15 @@ spec:
 
   post {
     always {
+      script {
+        if (fileExists('build/junit.xml')) {
+          try {
             junit allowEmptyResults: true, testResults: 'build/junit.xml'
+          } catch (Throwable err) {
+            echo "junit step unavailable: ${err.class.simpleName}"
+          }
+        }
+      }
       archiveArtifacts artifacts: 'build/junit.xml,build/coverage.json', allowEmptyArchive: true, fingerprint: true
       script {
         def props = fileExists('build.env') ? readProperties(file: 'build.env') : [:]