ci(release.yml): fetch tags to fix canaryExists logic

[vdice]

Previously, tags weren't actually fetched, throwing off the canaryExists logic.

Interestingly, tags still won't be fetched when fetch-depth is > 0 (as is the default, being 1), even when toggling fetch-tags: true on the checkout action. (See actions/checkout#701).

Therefore, I just add a git fetch --prune --unshallow --tags to start the step. (Prefer not to set fetch-depth to 0 and thus fetch all git history.)

Depends on #64 (will convert from Draft once merged)