Run builds in series

Since they I/O bound off of the same hardware, and sometimes also CPU
bound when compiling things, running them in parallel only slows
things down.

Closes: #28

pipelines/jobs/rust-xwin.yml.j2

@@ -16,12 +16,10 @@
 
 {% macro jobs() -%}
   - name: rust-xwin
+    serial: true
     plan:
       - get: repo
         trigger: true
-      - in_parallel:
-          fail_fast: true
-          steps:
       - task: build-rust-xwin
         file: repo/tasks/build-image.yml
         privileged: true

pipelines/jobs/simple-image.yml.j2

@@ -1,5 +1,6 @@
 {% macro jobs(job) -%}
   - name: {{ job.name }}
+    serial: true
     plan:
       - get: repo
         trigger: true