[ci] Publish rolling release build artifacts with stable URLs

[ci] Publish rolling release build artifacts with stable URLs

Publish the binaries built from commits on the master branch under
stable URLs such as:

  https://github.com/ipxe/ipxe/releases/download/rolling/bin/undionly.kpxe

Since filenames such as "ipxe.iso" may exist in each of several build
directories, we implement this as one release tag per build directory.
The GitHub Actions workflow automatically moves the tag to the most
recent commit and overwrites the existing release assets.

One downside of this is that running a local "git log" or similar may
show a large number of uninformative tags of the form "rolling/bin",
"rolling/bin-x86_64-efi", "rolling-arm64-efi", etc, all pointing at
the most recent commit.  This clutter may be hidden using:

  git config --local log.excludeDecoration refs/tags/rolling/*

To avoid the unintentional creation of rolling release tags on forks,
we skip the whole publication job unless the environment variable
ROLLING_PREFIX is defined.

Signed-off-by: Michael Brown <mcb30@ipxe.org>