build depends fresh if no existing tag to update

[infotroph]

(moved from robkooper#82)

Should fix Docker build failure in release branch that arises because the existing code tries to test-build the depends image by updating an existing pecan/depends:<base branch> and we (1) don't have a depends:master (we were calling it depends:latest), and (2) if we did, it would usually be so out of date it makes more sense to build a fresh image than to update it.

This was one of two approaches I proposed. See https://github.com/robkooper/pecan/pull/79 for the other and speak up if you like that better, but I now prefer this one.

Note that this will change behavior of non-PR calls to the Docker action:

Status quo:

• Non-PR Action runs with dependency changes do not build depends (which may or may not cause other failures)
• PRS with dependency changes try to build depends (and fail if no tag matches the base ref)

robkooper#79

• Non-PR Action runs with dependency changes do not build depends (which may or may not cause other failures)
• PRS with dependency changes try to build depends (and fall back to a clean build if no tag matches the base ref)

This PR:

• Non-PR Action runs with dependency changes build depends from scratch
• PRS with dependency changes try to build depends (and fall back to a clean build if no tag matches the base ref)

Invariant across all 3 approaches: Runs with no dependency changes do not build depends, whether or not they're a PR.