dockerbuild workflow のみキャッシュを保存し、テスト実行時はキャッシュを保存しない

[nanasess]

最初に実行される dockerbuild workflow のみ docker build のキャッシュを保存し、後続の workflow ではキャッシュの取得のみ行う。

若干の高速化と、以下のようなキャッシュのエラーを防止できる
https://github.com/EC-CUBE/ec-cube2/actions/runs/8384754767/job/22962608070#step:3:2232

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.21%. Comparing base (33f507d) to head (defc1df).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #870   +/-   ##
=======================================
  Coverage   55.21%   55.21%           
=======================================
  Files          75       75           
  Lines        8913     8913           
=======================================
  Hits         4921     4921           
  Misses       3992     3992           

Flag	Coverage Δ
tests	55.21% <ø> (ø)

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