[#1046] Read the latest release with the workflow token, and fail the step when it comes back empty

[vharseko]

Fixes #1046

What fails

Run 34854564649 (#968), job build-docker: the step «Get latest release version» printed last release: with nothing after it, docker/metadata-action warned «No Docker image version has been generated», and the push failed with ERROR: failed to build: tag is needed when pushing to registry. The maven legs of the run were green and attempt 2 passed with no change to the branch.

The step reads api.github.com/repos/OpenIdentityPlatform/OpenDJ/releases/latest with an anonymous curl. The anonymous limit is 60 requests an hour per source address and is shared by every job the runner's address hosts; a rate-limited answer carries no "name", so the version is empty and the image has no tag. build-docker-alpine reads the release the same way.

The change

.github/workflows/build.yml, the «Get latest release version» step of build-docker and build-docker-alpine:

• the step is given GH_TOKEN: ${{ github.token }} through env: — the shape the Windows-artifact wait already uses — and the curl sends it as Authorization: Bearer. The workflow token has a budget of its own; permissions: contents: read, which both jobs already declare, is enough to read the releases of a public repository, from a fork's read-only PR token included;
• when the version is still empty the step stops with ::error::The latest release of OpenIdentityPlatform/OpenDJ could not be read, so there is no version to tag the image with and exit 1, so the next failure of this shape is reported at the step which caused it rather than at the push.

The read itself (curl -i | grep "name" | cut) and everything after it are as they were.

Checked

• The workflow parses; both jobs carry the env and the new run.
• The step's shell, run locally with a token in GH_TOKEN: last release: 5.1.2, the guard passes. The header is honoured: the authenticated call answers x-ratelimit-limit: 5000, the anonymous one 60.
• The guard with an empty value prints the ::error and exits 1.
• The build-docker and build-docker-alpine legs of this PR's own run exercise the step on a runner.