Ignore unset variables from minikube docker-env

[maximdeclercq]

Description
In case shell is set to none, minikube docker-env returns SSH_AUTH_SOCK=.
This conflicts with the SSH_AUTH_SOCK variable set by the shell here.
Effectively, this allows users to use ssh: default again when building docker containers for minikube with skaffold.

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[codecov]

Codecov Report

Merging #9018 (0213154) into main (290280e) will decrease coverage by 6.87%.
Report is 998 commits behind head on main.
The diff coverage is 49.80%.

@@            Coverage Diff             @@
##             main    #9018      +/-   ##
==========================================
- Coverage   70.48%   63.62%   -6.87%     
==========================================
  Files         515      624     +109     
  Lines       23150    31928    +8778     
==========================================
+ Hits        16317    20313    +3996     
- Misses       5776    10085    +4309     
- Partials     1057     1530     +473     

Files Changed	Coverage Δ
cmd/skaffold/app/cmd/completion.go	13.04% <0.00%> (-1.25%)	⬇️
cmd/skaffold/app/cmd/config/list.go	65.21% <ø> (ø)
cmd/skaffold/app/cmd/config/set.go	88.72% <ø> (ø)
cmd/skaffold/app/cmd/config/util.go	54.28% <ø> (ø)
cmd/skaffold/app/cmd/credits.go	100.00% <ø> (ø)
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/generate_pipeline.go	60.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_modules.go	65.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_profiles.go	66.66% <ø> (ø)
... and 40 more

... and 416 files with indirect coverage changes

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