ci: move build scratch off node runtime storage

Jenkinsfile

@@ -2,10 +2,15 @@ pipeline {
   agent {
     kubernetes {
       defaultContainer 'tester'
+      // Build scratch belongs on storage volumes, not the node runtime USB drive.
+      workspaceVolume dynamicPVC(accessModes: 'ReadWriteOnce', requestsSize: '20Gi', storageClassName: 'ci-scratch')
       yaml """
 apiVersion: v1
 kind: Pod
 spec:
+  securityContext:
+    fsGroup: 1000
+    fsGroupChangePolicy: OnRootMismatch
   nodeSelector:
     kubernetes.io/arch: arm64
     node-role.kubernetes.io/worker: "true"
@@ -72,12 +77,17 @@ spec:
   volumes:
     - name: docker-config-writable
       emptyDir: {}
-    - name: workspace-volume
-      emptyDir: {}
 """
     }
   }
   environment {
+    PIP_CACHE_DIR = '/home/jenkins/agent/.cache/pip'
+    NPM_CONFIG_CACHE = '/home/jenkins/agent/.cache/npm'
+    SONAR_USER_HOME = '/home/jenkins/agent/.cache/sonar'
+    TMPDIR = '/home/jenkins/agent/.cache/tmp'
+    GOCACHE = '/home/jenkins/agent/.cache/go-build'
+    GOMODCACHE = '/home/jenkins/agent/.cache/go-mod'
+    GOTMPDIR = '/home/jenkins/agent/.cache/go-tmp'
     SUITE_NAME = 'soteria'
     PUSHGATEWAY_URL = 'http://platform-quality-gateway.monitoring.svc.cluster.local:9091'
     SONARQUBE_HOST_URL = 'http://sonarqube.quality.svc.cluster.local:9000'
@@ -104,6 +114,11 @@ spec:
     pollSCM('H/5 * * * *')
   }
   stages {
+    stage('Prepare build scratch') {
+      steps {
+        sh 'mkdir -p /home/jenkins/agent/.cache/tmp /home/jenkins/agent/.cache/go-tmp'
+      }
+    }
     stage('Checkout') {
       steps {
         checkout scm
@@ -114,6 +129,7 @@ spec:
         container('tester') {
           sh '''
             set -eu
+            mkdir -p "$GOCACHE" "$GOMODCACHE" "$GOTMPDIR"
             apt-get update >/dev/null
             apt-get install -y --no-install-recommends bash git jq curl python3 ripgrep >/dev/null
           '''