Support osversion when selecting base images

[imjasonh]

Fixes #534

This builds on #533 to add an option to the --platform flag to explicitly select a platform by osversion, which happens to be necessary to select the correct golang:1.17 base image for the GitHub Actions runner environment.

[codecov-commenter]

Codecov Report

Merging #536 (606c4c7) into main (2ba70fc) will decrease coverage by 0.04%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##             main     #536      +/-   ##
==========================================
- Coverage   48.16%   48.12%   -0.05%     
==========================================
  Files          42       42              
  Lines        2182     2184       +2     
==========================================
  Hits         1051     1051              
- Misses        954      955       +1     
- Partials      177      178       +1     

Impacted Files	Coverage Δ
pkg/commands/config.go	56.25% <62.50%> (-1.13%)	⬇️
pkg/commands/resolver.go	31.55% <66.66%> (-0.34%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2ba70fc...606c4c7. Read the comment docs.

[imjasonh]

Experimenting with the platform syntax used in https://github.com/estesp/mquery where platform is os[/arch[/variant]][:osversion], which would allows us to 1.) get away without a new flag 🎉 , and 2.) allow specifying multiple different osversions in --platform 🎉 🎉

This works:

$ go run ./ build ./ --platform=windows/amd64:10.0.17763.2300
...
gcr.io/imjasonh/ko-98b8c7facdad74510a7cae0cd368eb4e@sha256:65977075f677a3ead974261f523d8ffff894074695e5b65b13ff9d19a820cc66

This currently panicks:

$ go run ./ build ./ --platform=windows/amd64:10.0.17763.2300,windows/amd64:10.0.14393.4770
<snip, see https://github.com/google/ko/issues/544>

I'll work on figuring that out while we decide if we like this syntax.

If we go with this format we should probably also get crane to use it, and drop its --osversion flag too.

[imjasonh]

Hey, good news, this isn't a new panic introduced by this PR 😅 #544

[imjasonh]

Any more concerns with this?