fix: read full packs from cold backend in hot/cold case

[aawsome]

closes rustic-rs/rustic#1811

As a side effect, checking packs data now reads the blobs in chunks leading to less memory usage.

[codecov]

Codecov Report

❌ Patch coverage is 73.91304% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.1%. Comparing base (304bb86) to head (66adec7).

Files with missing lines	Patch %	Lines
crates/core/src/commands/check.rs	71.4%	12 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/crypto/hasher.rs	73.3% <100.0%> (+23.3%)	⬆️
crates/core/src/commands/check.rs	71.9% <71.4%> (-0.9%)	⬇️

... and 21 files with indirect coverage changes

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