Re-exec gcloud token source when we refresh

[jonjohnsonjr]

Previously, this was storing a single exec.Command, but that gets
consumed when it gets Run, failing with:

exec: already started

This will re-exec the gcloud when needed.

[codecov-commenter]

Codecov Report

Merging #1017 (baae9ab) into main (4c244d6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1017   +/-   ##
=======================================
  Coverage   75.22%   75.22%           
=======================================
  Files         107      107           
  Lines        5053     5053           
=======================================
  Hits         3801     3801           
  Misses        706      706           
  Partials      546      546           

Impacted Files	Coverage Δ
pkg/v1/google/auth.go	64.58% <100.00%> (ø)

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