Avoid race in progress_test.go

[imjasonh]

$ go test -race ./pkg/v1/remote -run=TestMultiWrite_Progress_Retry -count=10
ok  	github.com/google/go-containerregistry/pkg/v1/remote	60.598s

[codecov-commenter]

Codecov Report

Merging #1081 (b9f090f) into main (03f1bf4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1081   +/-   ##
=======================================
  Coverage   75.50%   75.50%           
=======================================
  Files         107      107           
  Lines        5094     5094           
=======================================
  Hits         3846     3846           
  Misses        703      703           
  Partials      545      545           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 03f1bf4...b9f090f. Read the comment docs.