perf: keep the emulator out of the qemu-user package for later builds

[The127]

A cross build used to unpack the qemu-user package on every run to find one program. On an arm64 host the amd64 program sits near the end of the package, so most of it was decompressed each time.

Now the first cross build keeps the program in the download store under its pinned digest. Later builds read it from there. A program that does not match its pin is refused and removed. miso prune treats the kept program like any other download.

Docs: I checked them. Nothing needed a change.

[codecov]

Codecov Report

❌ Patch coverage is 64.51613% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.12%. Comparing base (5196ee1) to head (3f7f580).

Files with missing lines	Patch %	Lines
internal/download/downloadtest/kept.go	0.00%	9 Missing ⚠️
internal/download/pinned.go	90.00%	1 Missing ⚠️
internal/emulator/ready.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   56.10%   56.12%   +0.02%     
==========================================
  Files         288      288              
  Lines        4761     4780      +19     
==========================================
+ Hits         2671     2683      +12     
- Misses       2090     2097       +7     

Flag	Coverage Δ
unit	56.12% <64.51%> (+0.02%)	⬆️

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[The127]

Closing: miso will boot a builder VM of the target architecture with the host's qemu-system instead of downloading qemu-user.