fix: Fix a possible race condition in metadata retrieval

[jannfis]

This fixes a possible race condition when fetching tag metadata from a registry.

Should fix #214, however I am not able to reproduce the original issue.

Signed-off-by: jannfis jann@mistrust.net

[codecov]

Codecov Report

Merging #215 (1f304d8) into master (45e6486) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   66.77%   66.77%           
=======================================
  Files          20       20           
  Lines        1574     1574           
=======================================
  Hits         1051     1051           
  Misses        424      424           
  Partials       99       99           

Impacted Files	Coverage Δ
pkg/registry/registry.go	59.81% <100.00%> (ø)

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