[tor-bugs] #26106 [Core Tor/Tor]: bump rand dependency to 0.5.0-pre.2

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#26106: bump rand dependency to 0.5.0-pre.2
------------------------------+--------------------------------
     Reporter:  isis          |      Owner:  isis
         Type:  defect        |     Status:  assigned
     Priority:  Medium        |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  rust
Actual Points:                |  Parent ID:
       Points:  .1            |   Reviewer:
      Sponsor:  SponsorV-can  |
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 This is blocking #23886, because the `rand_core::RngCore` and `rand::Rng`
 traits resolve as being different, because `curve25519-dalek` needs to use
 `0.5.0-pre.2` because it added `impl CryptoRng for OsRng` (which we also
 probably want but I did a workaround in #25508).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26106>
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