Fixes prepare_release.sh.

[coollog]

Updates from fix at GoogleContainerTools/jib#1129 (comment)

[codecov]

Codecov Report

Merging #335 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #335   +/-   ##
=========================================
  Coverage     60.49%   60.49%           
  Complexity      132      132           
=========================================
  Files            28       28           
  Lines           605      605           
  Branches         56       56           
=========================================
  Hits            366      366           
  Misses          210      210           
  Partials         29       29

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