rust: Consolidate cargo environment variables

This consolidates all environment variables for cargo into:

* CARGO_HOST_CONFIG_VARS / CARGO_PKG_CONFIG_VARS

  These contain all cargo-specific environment variables, i.e. without
  "common" variables like CC.

* CARGO_HOST_VARS / CARGO_PKG_VARS (renamed from CARGO_VARS)

  These contain all environment variables to be passed to cargo.

This also:

* Set the CARGO_BUILD_TARGET environment variable instead of using the
  --target command-line option

* Update Python include files to use CARGO_HOST_CONFIG_VARS /
  CARGO_PKG_CONFIG_VARS

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

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