Make build workflows triggerable via dispatch with commitish input

[jwodder]

As requested in #8.

[jwodder]

The option to trigger the workflows manually isn't showing up in the web UI (presumably because the PR isn't merged), but I was able to trigger this workflow run via the REST API:

curl -XPOST \
    -H "Accept: application/vnd.github.v3+json" \
    -H "Authorization: REDACTED" \
    -d '{"ref": "gh-8", "inputs": {"commitish": "8.20200908"}}' \
    https://api.github.com/repos/datalad/git-annex/actions/workflows/3364464/dispatches

(I tried some older tags first, but those builds failed because the repo then didn't have a certain file the workflow needs.)