Makefile: avoid ?= shell executions

[joelanford]

Description of the change:
Avoid ?= shell executions

Motivation for the change:
They are re-evaluated every time they are referenced and significantly slow down make invocations.

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: joelanford

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [joelanford]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.02%. Comparing base (da4828d) to head (70c1dbf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1328   +/-   ##
=======================================
  Coverage   50.02%   50.02%           
=======================================
  Files         135      135           
  Lines       13006    13006           
=======================================
  Hits         6506     6506           
  Misses       5416     5416           
  Partials     1084     1084           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[grokspawn]

/lgtm