Allow for overriding how trigger release announcement step gets version information

[SeanTAllen]

Prior to this commit, the trigger release announcement step
gets the version from GITHUB_REF. For this to work, the step
has to be triggered by a tag being pushed.

This is good for most use-cases, but not all. For example, ponyc
build assets for release on cirrus-ci not github actions. This
means that we want to trigger a release announcement once we've
been notified assets have been uploaded to cloudsmith.

This will not work with trigger release announcement as it can't
get the version from GITHUB_REF as it isn't available there.

This commit adds the ability to provide an optional environment
variable VERSION to the trigger-release-announcement step. If the
variable is set, the version will be taken from that rather than
trying to parse it out of GITHUB_REF.

Closes #22

[ponylang-main]

Hi @SeanTAllen,

The changelog - added label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 23.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[Theodus]

LGTM