Support partial platform matching for osversion only

[imjasonh]

Also move platform filtering into one location in pkg/build/gobuild.go. If only one child image matches, fallback to building a single-platform image; otherwise, build a (possibly filtered) multi-platform index.

A lot of this code will benefit from platform (de-)stringification improving inside ggcr, which I'll send out shortly.

[codecov-commenter]

Codecov Report

Merging #572 (e6211d0) into main (70b671c) will decrease coverage by 0.16%.
The diff coverage is 59.37%.

@@            Coverage Diff             @@
##             main     #572      +/-   ##
==========================================
- Coverage   48.70%   48.53%   -0.17%     
==========================================
  Files          43       43              
  Lines        2234     2215      -19     
==========================================
- Hits         1088     1075      -13     
+ Misses        959      954       -5     
+ Partials      187      186       -1     

Impacted Files	Coverage Δ
pkg/commands/config.go	54.54% <33.33%> (-3.04%)	⬇️
pkg/build/gobuild.go	57.25% <62.06%> (-0.15%)	⬇️

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