Add retries for the sapling and sprout parameter downloads

[teor2345]

Motivation

A build of PR #3191 failed with:

Dec 15 21:55:10.404 INFO {zebrad="de6b673" net="Main"}: zebra_consensus::primitives::groth16::params: downloading Zcash Sapling parameters
Dec 15 21:56:15.162 INFO {zebrad="de6b673" net="Main"}: zebra_consensus::primitives::groth16::params: downloading Zcash Sprout parameters
The application panicked (crashed).
Message: error downloading Sprout parameter files: IoError(Custom { kind: InvalidData, error: "sprout-groth16.params failed validation: expected: e9b238411bd6c0ec4791e9d04245ec350c9c5744f5610dfcce4365d5ca49dfefd5054e371842b3f88fa1b9d7e8e075249b3ebabd167fa8b0f3161292d36c180a hashing 725523612 bytes, actual: 980ecfb84bef129ff5547a5462e29c2cbff91ecf717d17aec15c6d506a12297e28a8b1c7cac9e3faddce712d02575980e8d29ee232bba3e32f438c14c256e69f hashing 609221143 bytes from "https://download.z.cash/downloads/sprout-groth16.params.part.1 + https://download.z.cash/downloads/sprout-groth16.params.part.2\"" }). Hint: try deleting "/root/.zcash-params", then running 'zebrad download' to re-download the parameters
Location: zebra-consensus/src/primitives/groth16/params.rs:88

Designs

• Add a retry to Zebra's parameter downloads
• Only retry the downloads that actually failed

zebra/zebra-consensus/src/primitives/groth16/params.rs

Line 71 in ea01e30

zcash_proofs::download_sapling_parameters(Some(PARAMETER_DOWNLOAD_TIMEOUT))

zebra/zebra-consensus/src/primitives/groth16/params.rs

Line 86 in ea01e30

zcash_proofs::download_sprout_parameters(Some(PARAMETER_DOWNLOAD_TIMEOUT))

[mpguerra]

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[teor2345]

This caused another CI build to fail, so I'm moving it to this sprint.