Disable security with downloading sccache tarball

Some organizations use a firewall that intercepts SSL communications and replaces the authentication token with an organization-wide token that is just supposed to be accepted. Browsers can be locally configured for this intentional trickery, but it's a little more tricky for the use of curl in a batch script that is shared with external collaborators.

The easiest solution is to just disable the security for the curl download. This makes the script more susceptible to "man in the middle" attacks, but it's probably easier to just slip malware in the public repos anyway.