Consider HEAD to blob before upload

[vsoch]

Currently we just try pushing a blob and don't check if it already exists. I tried adding this to #32 but hit some bugs that I didn't have bandwidth to work through (and it was adding too much complexity / too many variables to the PR), so I'm creating an issue as a reminder this is worth trying again.

We probably also need to set up some basic tests that are allowed to push to a GitHub packages registry to test that, not sure how others feel about allowing a push during a test (but we can discuss!)