This PR adds the ability to upload release artifact via XCHammer's github
CI.

On each build, we upload the binary release artifact `xchammer.zip`
which can be pulled into a bazel `WORKSPACE` with `http_archive` and
then used as expected.

At first, I'd considered creating a release on each commit but that has
a few complexities in github actions e.g. can't create a release w/o a
tag, and would end up annoying watchers of the repository with a release
email on each commit landed. Perhaps there could be a better way to navigate
to these artifact URLs in the github UI longer term.

This reverts commit 4dc05e6.

The github actions API doesn't play well with per commit artifacts. For
now we create a release when pushing a tag.

Consuming the per commit artifacts w/o the github actions API is blocked
by this: actions/upload-artifact#50