Fragile automated builds on lizard

Originally created by @intrigeri on #10334 (Redmine)

We’ve seen many build failures recently. Almost all of them happen on isobuilder2, I had a look and it’s not surprising things fail:

$ df
df: `/tmp/tails-build.4ZSfX1Cx': Stale NFS file handle
df: `/tmp/tails-build.9zHHWL68': Stale NFS file handle
[...]
tmpfs                      tmpfs     8.8G  5.6G  3.3G  64% /tmp/tmpfs.MtxhVedc
tmpfs                      tmpfs     8.8G  782M  8.1G   9% /tmp/tmpfs.NQz5b14m

$ free
             total       used       free     shared    buffers     cached
Mem:          8992       6962       2029          0         10       6487
-/+ buffers/cache:        464       8527
Swap:            0          0          0

So there are two problems there:

• I don’t know what the NFS error come from, I see a NFS line in fstab, I thought we were not supposed to have any such thing anymore, but perhaps I’m confused.
• The build wrapper doesn’t always clean up properly.

I’ve rebooted isobuilder2 so the avalanche of notifications should stop.

Related issues

• Related to #6090 (closed)
• Related to tails#10772 (closed)