Rely on `WORKDIR` env var to avoid ugly ternary operators in workflows. (#9456)

Ternary operators in GitHub Actions syntax are pretty ugly and hard to
read: `inputs.working-directory == '' && '.' ||
inputs.working-directory` means "if the condition is true, use `'.'` and
otherwise use the expression after the `||`".

This PR performs the ternary as few times as possible, assigning its
outcome to an env var we can then reuse as needed.