chore: make test env check output what was found

[briandealwis]

Little change to the testutil's assertCmdEnv function to output the actual environment variable values found. This function is used by RunCmd() and RunCmdOut().

[aaron-prindle]

LGTM. The errors should go away once rebased, were related to an issue fixed by - https://github.com/GoogleContainerTools/skaffold/pull/6752/files

[codecov]

Codecov Report

Merging #6744 (9035836) into main (290280e) will decrease coverage by 0.78%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##             main    #6744      +/-   ##
==========================================
- Coverage   70.48%   69.69%   -0.79%     
==========================================
  Files         515      536      +21     
  Lines       23150    24303    +1153     
==========================================
+ Hits        16317    16938     +621     
- Misses       5776     6249     +473     
- Partials     1057     1116      +59     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/deploy.go	52.00% <ø> (-1.85%)	⬇️
cmd/skaffold/app/cmd/flags.go	89.00% <0.00%> (-1.82%)	⬇️
cmd/skaffold/app/cmd/render.go	36.66% <0.00%> (-4.72%)	⬇️
cmd/skaffold/skaffold.go	0.00% <ø> (ø)
cmd/skaffold/app/cmd/lint.go	52.94% <52.94%> (ø)
cmd/skaffold/app/cmd/cmd.go	70.49% <75.00%> (-0.57%)	⬇️
cmd/skaffold/app/cmd/debug.go	100.00% <100.00%> (ø)
cmd/skaffold/app/cmd/runner.go	64.17% <100.00%> (ø)
cmd/skaffold/app/flags/image.go	76.27% <100.00%> (+3.72%)	⬆️
cmd/skaffold/app/skaffold.go	85.71% <100.00%> (+1.09%)	⬆️
... and 121 more

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