feat: add CARGO_MANIFEST_PATH env variable

[Ifropc]

Adds CARGO_MANIFEST_PATH variable as part of #12207
Context: CARGO_MANIFEST_DIR is not very useful, because there is no Cargo.toml file when running a cargo script. In cases when multiple scripts are stored in the same folder, we can't tell which script exactly is being run using CARGO_MANIFEST_DIR

[rustbot]

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[epage]

@rfcbot merge

This PR insta-stabilizes CARGO_MANIFEST_PATH, a companion to CARGO_MANIFEST_DIR for when you actually want the manifest because CARGO_MANIFEST_DIR.join("Cargo.toml") will no longer be sufficient due to cargo-script.

[rfcbot]

Team member @epage has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Eh2406
• @Muscraft
• @arlosi
• @ehuss
• @epage
• @joshtriplett
• @weihanglo

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