build: Consistently default to linux/amd64

[jsoref]

Checklist:

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.02 🎉

Comparison is base (4ca903b) 49.61% compared to head (28d35d5) 49.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14146      +/-   ##
==========================================
+ Coverage   49.61%   49.64%   +0.02%     
==========================================
  Files         256      256              
  Lines       43925    43914      -11     
==========================================
+ Hits        21793    21800       +7     
+ Misses      19975    19958      -17     
+ Partials     2157     2156       -1     

see 7 files with indirect coverage changes

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[jannfis]

LGTM, thanks!