Fix environment variable for destination repo

[agherzan]

Consolidate the variable as being: GMM_DST_REPO.

[codecov]

Codecov Report

Merging #19 (b761e77) into main (37fa8ec) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##             main      #19   +/-   ##
=======================================
  Coverage   75.44%   75.44%           
=======================================
  Files           7        7           
  Lines         566      566           
=======================================
  Hits          427      427           
  Misses        104      104           
  Partials       35       35           

Impacted Files	Coverage Δ
cmd/git-mirror-me/main.go	54.00% <0.00%> (ø)
cmd/git-mirror-me/args.go	100.00% <100.00%> (ø)

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