Make Github PAT optional when downloading binaries from public repositories

[jmcphers]

We currently require a Github PAT in order to download Positron's pre-built binaries (including Kallichore, ARK, and perhaps PET soon), despite the fact that these binaries are in public repositories that don't need a Github PAT to access.

It is still helpful to supply a PAT for e.g. rate limiting purposes, but requiring one needlessly complicates the process of getting Positron built on a new machine (especially for third-party contributors or others who do not need to build frequently).

The download scripts for these binaries should use a PAT if one is available, but proceed to download without one otherwise.