[racket] Permission errors with nightlies?

Background: https://travis-ci.org/greghendershott/frog/jobs/39956106

I'm using http://www.cs.utah.edu/plt/snapshots/current/installers/racket-current-x86_64-linux-precise.sh
on Travis CI.

Suddenly I'm seeing dozens of error like this:

open-output-file: cannot open output file
  path: /usr/racket/share/pkgs/syntax-color-lib/syntax-color/compiled/lexer-contract_rkt.zo
  system error: Permission denied; errno=13
  context...:
   /usr/racket/collects/compiler/cm.rkt:185:4
   /usr/racket/collects/compiler/cm.rkt:470:0: maybe-compile-zo
   /usr/racket/collects/compiler/cm.rkt:581:2: do-check
   /usr/racket/collects/compiler/cm.rkt:620:15
   /usr/racket/collects/compiler/../racket/private/map.rkt:113:23: loop
   /usr/racket/collects/compiler/cm.rkt:581:2: do-check
   /usr/racket/collects/compiler/cm.rkt:620:15
   /usr/racket/collects/compiler/../racket/private/map.rkt:113:23: loop
   /usr/racket/collects/compiler/cm.rkt:581:2: do-check
   /usr/racket/collects/compiler/cm.rkt:226:11: for-loop
   /usr/racket/collects/compiler/cm.rkt:470:0: maybe-compile-zo
   /usr/racket/collects/compiler/cm.rkt:581:2: do-check
   /usr/racket/collects/compiler/cm.rkt:696:4: compilation-manager-load-handler
   standard-module-name-resolver
   /usr/racket/collects/compiler/cm.rkt:309:0: compile-zo*
   /usr/racket/collects/compiler/cm.rkt:512:26...

ISTR this happening a month or two ago, and it resolved itself on the
next nightly build.

Now that it's happened again, I wanted to ask why it happens?

I like to have Travis CI build my projects against HEAD (nightlies) as
well as released versions. I understand that nightlies might be buggy.
So one answer might be, I should stop doing this. On the other hand I
like the idea of an "early alert" if some change to Racket isn't
compatible with something I'm doing. And 99% of the time it works
fine. So I guess I'm asking about this specific 1% case, now that it's
recurred.