Add [env] section to Cargo Reference - Configuration

[flxo]

With #9175 a new feature landed in nightly cargo:

An [env] section in the cargo configuration to pass additional env variables to all program invocations does by cargo.

From the PR:

[env]
FOOBAR = "Apple"
PATH_TO_SOME_TOOL = { value = "bin/tool", relative = true }
USERNAME = { value = "test_user", force = true }

This information is missing in the Cargo Reference/Configuration section.

[flxo]

Sorry - just noticed that the cargo book is hosted here