ci: download snap to tests dir

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

The scheduled spread tests were not downloading the snap to the tests/ directory.

I triggered a run and it is working now. See logs here: https://github.com/snapcore/snapcraft/actions/runs/6313456368

[cmatsuoka]

Thanks!

[codecov-commenter]

Codecov Report

Merging #4380 (98242a1) into main (0544bd3) will decrease coverage by 0.01%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main    #4380      +/-   ##
==========================================
- Coverage   89.12%   89.11%   -0.01%     
==========================================
  Files         318      318              
  Lines       21174    21174              
==========================================
- Hits        18871    18870       -1     
- Misses       2303     2304       +1     

see 1 file with indirect coverage changes

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