Add a workflow to push a :latest ko image to ghcr.io

[imjasonh]

This also tags images with the commit SHA.

In my fork:

• run: https://github.com/imjasonh/ko/runs/4473935758?check_suite_focus=true
• package UI: https://github.com/imjasonh/ko/pkgs/container/ko
• image: ghcr.io/imjasonh/ko:12ebaefe87c83c062df6a55c452d5fb247324bca

If this goes well we can also tag images at release-time.

Not documenting in the README in this PR since I want to get more miles on it before we recommend it to people.

[codecov-commenter]

Codecov Report

Merging #528 (5d2b992) into main (ad0607f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #528   +/-   ##
=======================================
  Coverage   48.11%   48.11%           
=======================================
  Files          42       42           
  Lines        2170     2170           
=======================================
  Hits         1044     1044           
  Misses        950      950           
  Partials      176      176           

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[imjasonh]

After these edits: https://github.com/imjasonh/ko/runs/4474138554?check_suite_focus=true