Consider the interaction of the [env] table with Cargo's own environment

[ehuss]

Currently Cargo does not inspect the [env] config table for its own environment variables. This may have been an oversight and it is not clear exactly how to proceed from here. A few options:

• Merge the [env] table into Cargo's environment snapshot, and make cargo honor most of the environment variables.
  • There are some environment variables with subtle complexities like CARGO_HOME and PATH.
• Deny (or issue a warning) if you attempt to set one of Cargo's environment variables in the [env] table.
  • Unfortunately this will be challenging, and a breaking change. There are some users already setting environment variables that start with CARGO_ for their own purpose. Also, those environment variables will apply to recursive cargo invocations.

Related issues:

• cargo subcommands do not pick up [env] #10094: Should cargo at least set the environment for external subcommands?
• fix: make subcommands pick up [env] #10780: Attempt to forbid setting CARGO_ environment variables (among other things).