9564d073b9
`${GITHUB_SHA:0:7}` substring expansion is bash-only; the runner
executes the step via dash/BusyBox sh and errored with
`Bad substitution` (act_runner workflow shell, observed on the
65386f0 main-push run). Switched to
`SHORT_SHA=$(printf '%s' "$GITHUB_SHA" | cut -c1-7)` which works in
both shells. Both build-web and build-ml tag-determination steps
updated.