Use parsed ssh username if git2go does not provide one

[tomkennedy513]

This is required because git2go is unable to parse the username when calling remote.Fetch with the go ssh transport

[codecov-commenter]

Codecov Report

Merging #1121 (be3f613) into main (3e3e489) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1121      +/-   ##
==========================================
+ Coverage   69.85%   69.97%   +0.12%     
==========================================
  Files         124      124              
  Lines        6193     6219      +26     
==========================================
+ Hits         4326     4352      +26     
  Misses       1462     1462              
  Partials      405      405              

Impacted Files	Coverage Δ
pkg/git/git_keychain.go	82.43% <100.00%> (+0.48%)	⬆️
pkg/apis/build/v1alpha1/builder_validation.go	100.00% <0.00%> (ø)
pkg/apis/build/v1alpha2/builder_validation.go	100.00% <0.00%> (ø)

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[chenbh]

Can we add either a code comment or commit message that explains the rationale behind this change? I fear the context is going to be lost especially since we don't have an issue explaining the problem.