wip: can we cache mix?

Author: janl

build-aux/Jenkinsfile

@@ -498,7 +498,6 @@ pipeline {
   }
 
   stages {
-
     stage('Setup Env') {
       agent {
         docker {
@@ -612,13 +611,23 @@ pipeline {
         }
       }
       steps {
-        sh '''
-          rm -rf apache-couchdb-*
-          ./configure --skip-deps --spidermonkey-version 78
-          make erlfmt-check
-          make elixir-source-checks
-          make python-black
-        '''
+        cache(
+          maxCacheSize: 250,
+          defaultBranch: 'main',
+            caches: [
+              arbitraryFileCache(
+                path: '_build',
+                compressionMethod: 'TAR', // for binaries
+                cacheValidityDecidingFile: 'mix.exs')
+          ]) {
+          sh '''
+            rm -rf apache-couchdb-*
+            ./configure --skip-deps --spidermonkey-version 78
+            make erlfmt-check
+            make elixir-source-checks
+            make python-black
+          '''
+        }
       }
       post {
         cleanup {