[antlir2][snapshot] be more tolerant of slow mirror

Summary:
We need to be more tolerant of slow downloads that will eventually succeed -
mirrors may throttle us or might just be slow at baseline. Taking longer to
download the long tail of packages is better than just failing the whole thing.

___

Test Plan:
It now handles very large packages with slow download times on a real snapshot:
```
❯ buck2 run fbcode//mode/opt fbcode//antlir/antlir2/package_managers/snapshot:snapshot -- snapshot fbcode//antlir/antlir2/package_managers/deb:noble
```
https://pxl.cl/cvpjb

And the unit tests to explicitly test for pathological behavior
```
❯ buck2 test fbcode//antlir/antlir2/package_managers/snapshot/...
Buck UI: https://www.internalfb.com/buck2/7070232d-a06a-48a0-82d6-d9c5a065b042
Tests finished: Pass 33. Fail 0. Timeout 0. Fatal 0. Skip 0. Omit 0. Infra Failure 0. Build failure 0
```
https://www.internalfb.com/intern/testinfra/testrun/27866022734680339

Differential Revision: D116860677

fbshipit-source-id: 5316d5500a9bf2b1e227a7548c3a880ac47a5b00

Author: vmagro