pipeline: stop using /srv as workdir

I think we should stop using `/srv` as a workdir entirely and just
always build in the workspace. The core issue here is that (1) we want
to be able to have concurrent builds, and (2) a cosa workdir can't be
easily shared today. This also simplifies the devel vs prod logic quite
a bit since it had some funky conditionals around this.

So then, how can developers without S3 creds actually *access* built
artifacts? We simply archive them as part of the build. This is in line
also with coreos#31, where we'll probably be archiving things anyway.

Finally, how *can* we use the PVC as cache in a safe way shareable
across all the streams? I see two options offhand:
1. as a local RPM mirror: add flags and logic to `cosa fetch` to read &
   write RPMs in `/srv`, hold a lock to regen metadata
2. as a pkgcache repo: similarly to the above, but also doing the
   import, so it's just a pkgcache repo; this would probably require
   teaching rpm-ostree about this, or `cosa fetch` could just blindly
   import every ref

Jenkinsfile

@@ -54,15 +54,6 @@ if (prod) {
 podTemplate(cloud: 'openshift', label: 'coreos-assembler', yaml: pod, defaultContainer: 'jnlp') {
     node('coreos-assembler') { container('coreos-assembler') {
 
-        // Only use the PVC for prod caching. For devel pipelines, we just
-        // always refetch from scratch: we don't want to allocate cached data
-        // for pipelines which may only run once.
-        if (prod) {
-          utils.workdir = "/srv"
-        } else {
-          utils.workdir = env.WORKSPACE
-        }
-
         // this is defined IFF we *should* and we *can* upload to S3
         def s3_builddir
 
@@ -80,13 +71,6 @@ podTemplate(cloud: 'openshift', label: 'coreos-assembler', yaml: pod, defaultCon
           }
         }
 
-        // Special case for devel pipelines not running in our project and not
-        // uploading to S3; in that case, the only way to make the builds
-        // accessible at all is to have them in the PVC.
-        if (!prod && !prod_jenkins && !s3_builddir) {
-            utils.workdir = "/srv"
-        }
-
         stage('Init') {
 
             def ref = params.STREAM
@@ -162,22 +146,6 @@ podTemplate(cloud: 'openshift', label: 'coreos-assembler', yaml: pod, defaultCon
             """)
         }
 
-        stage('Prune Cache') {
-            utils.shwrap("""
-            coreos-assembler prune --keep=1
-            """)
-
-            // If the cache img is larger than e.g. 8G, then nuke it. Otherwise
-            // it'll just keep growing and we'll hit ENOSPC.
-            utils.shwrap("""
-            if [ \$(du cache/cache.qcow2 | cut -f1) -gt \$((1024*1024*8)) ]; then
-                rm -vf cache/cache.qcow2
-                qemu-img create -f qcow2 cache/cache.qcow2 10G
-                LIBGUESTFS_BACKEND=direct virt-format --filesystem=xfs -a cache/cache.qcow2
-            fi
-            """)
-        }
-
         stage('Archive') {
 
             // First, compress image artifacts
@@ -191,6 +159,11 @@ podTemplate(cloud: 'openshift', label: 'coreos-assembler', yaml: pod, defaultCon
               utils.shwrap("""
               coreos-assembler buildupload s3 --acl=public-read ${s3_builddir}
               """)
+            } else if (!prod) {
+              // In devel mode without an S3 server, just archive in Jenkins
+              // itself. Otherwise there'd be no other way to retrieve the
+              // artifacts.
+              archiveArtifacts('builds/latest/*')
             }
 
             // XXX: For now, we keep uploading the latest build to the artifact

utils.groovy

@@ -1,25 +1,20 @@
-workdir = env.WORKSPACE
-
 def shwrap(cmds) {
     sh """
         set -xeuo pipefail
-        cd ${workdir}
         ${cmds}
     """
 }
 
 def shwrap_capture(cmds) {
     return sh(returnStdout: true, script: """
         set -euo pipefail
-        cd ${workdir}
         ${cmds}
     """).trim()
 }
 
 def shwrap_rc(cmds) {
     return sh(returnStatus: true, script: """
         set -euo pipefail
-        cd ${workdir}
         ${cmds}
     """)
 }