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#### Why I did it

When building a Bullseye-based container with MIRROR_SNAPSHOT=y, the repos used will point to the main Debian repo, but Bullseye packages have been moved to the archive repo. Fix this issue by overriding the `DEFAULT_MIRROR_URLS` variable.

Note that bullseye-security is still in the main Debian repo, and is not yet on the archive repo.

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it

#### How to verify it

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#### Which release branch to backport (provide reason below if selected)

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- [ ] 202205
- [ ] 202211
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [ ] 202411
- [ ] 202505

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- [ ] <!-- image version 1 -->
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#### Link to config_db schema for YANG module changes
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