fix build date format for version command

[cpanato]

Summary

• fix build date format for version command

xref: sigstore/cosign#1644

Ticket Link

n/a

Release Note

NONE

[codecov-commenter]

Codecov Report

Merging #484 (5eebd96) into main (374ebab) will decrease coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #484      +/-   ##
==========================================
- Coverage   47.78%   47.49%   -0.29%     
==========================================
  Files          14       14              
  Lines        1059     1059              
==========================================
- Hits          506      503       -3     
- Misses        480      482       +2     
- Partials       73       74       +1     

Impacted Files	Coverage Δ
pkg/ca/fileca/load.go	68.13% <0.00%> (-3.30%)	⬇️

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