Enable automatic release (publish C XLA lib)

[joelberkeley]

When I make changes to spidr, these changes should be immediately available to pack's next nightly build. We want this to happen without having to manually build and publish the C XLA build artifact*. There are several ways we can do this. We can host the artifact in a number of places

• GitHub releases in the spidr repo
• Files tracked by git in another dedicated repo
• Cloud storage e.g. GCP

How often do we plan on releasing?

• On every push to main
• On every push to main that affects the C XLA build artifact (which should be when we change the C++ code)?

* do we need to do this? How often do we plan on changing the C XLA code?

[joelberkeley]

done in #356