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Currently, Cargo will always set `$CARGO` to point to what it detects its own path to be (using `std::env::current_exe`). Unfortunately, this runs into trouble when Cargo is used as a library, or when `current_exe` is not actually the binary itself (e.g., when invoked through Valgrind or `ld.so`), since `$CARGO` will not point at something that can be used as `cargo`. This, in turn, means that users can't currently rely on `$CARGO` to do the right thing, and will sometimes have to invoke `cargo` directly from `$PATH` instead, which may not reflect the `cargo` that's currently in use. This patch makes Cargo re-use the existing value of `$CARGO` if it's already set in the environment. For Cargo subcommands, this will mean that the initial invocation of `cargo` in `cargo foo` will set `$CARGO`, and then Cargo-as-a-library inside of `cargo-foo` will inherit that (correct) value instead of overwriting it with the incorrect value `cargo-foo`. For other execution environments that do not have `cargo` in their call stack, it gives them the opportunity to set a working value for `$CARGO`. One note about the implementation of this is that the test suite now needs to override `$CARGO` explicitly so that the _user's_ `$CARGO` does not interfere with the contents of the tests. It _could_ remove `$CARGO` instead, but overriding it seemed less error-prone. Fixes #10119. Fixes #10113.