Add caching support to sandbox apps creation

[elia]

Summary

With a warm cache the sandbox app creation goes from ~130s to ~30s on my Apple M1.

The optimization is used for the dummy-app creation as well.

Checklist

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[codecov[bot]]

Codecov Report

Merging #272 (30644f7) into master (59f4403) will decrease coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
- Coverage   99.59%   99.56%   -0.03%     
==========================================
  Files          27       26       -1     
  Lines         489      462      -27     
==========================================
- Hits          487      460      -27     
  Misses          2        2              

see 1 file with indirect coverage changes

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