Makefile: avoid re-evaluating vars unnecessarily; other minor fixup

[joelanford]

Fixes #875

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.27%. Comparing base (f9cee1a) to head (ca4ce93).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #877   +/-   ##
=======================================
  Coverage   37.27%   37.27%           
=======================================
  Files           9        9           
  Lines         845      845           
=======================================
  Hits          315      315           
  Misses        486      486           
  Partials       44       44           

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[grokspawn]

I'm still not 100% sold on replacing ?= statements with the likes of

ifeq ($(origin IMAGE_REPO), undefined)
include .bingo/Variables.mk
IMAGE_REPO := quay.io/operator-framework/rukpak
endif

since according to gnu make these are equivalent. Any other behavior should be considered a bug in the tool.

However, as with the related PRs there are lots of other improvements which tangibly improve make interactions, so I'm content to agree-to-disagree on this point. 😄