bug#56893: rust-vergen inserts build timestamps, possible irreproducibility source

[Maxime Devos]

While fixing build failures in antioxidant, I noticed that rust-vergen 
is a potential source of irreproducibility -- the README.md contains the 
following:

> ## Documentation
> [Documentation](https://docs.rs/vergen)
>
> ## Generate Compile Time Information
> `vergen`, when used in conjunction with cargo [build scripts], will
> generate environment variables to use with the `env!` macro. Below
> is a list of the supported variables.
>
> Key                       | Sample Value
> --------------------------|----------------------------------------
> VERGEN_BUILD_TIMESTAMP    |2018-08-09T15:15:57.282334589+00:000
> VERGEN_BUILD_DATE         |2018-08-09
> VERGEN_SHA |75b390dc6c05a6a4aa2791cc7b3934591803bc22
> VERGEN_SHA_SHORT          |75b390d
> VERGEN_COMMIT_DATE        |2018-08-08
> VERGEN_TARGET_TRIPLE      |x86_64-unknown-linux-gnu
> VERGEN_SEMVER             |v3.0.0
> VERGEN_SEMVER_LIGHTWEIGHT |v3.0.0
I'll try patching out the timestamps with 1970-...

Greetings,
Maxime.

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