The nightly package name includes the date, and so we use a glob to
locate the package file without hard coding a value. However, globbing
will not work with our Buildbot steps setup because we perform word
splitting ourselves and pass an array to Buildbot, which will directly
exec the array instead of passing it to the shell, meaning globbing
does not occur. Instead, add a script to the servo repo where we can
use globbing, and use `shopt -s failglob` to guard against bad globs.