Avoid console output for cargo script with unchanged builds

[Timmmm]

This adds a flag that records whether any work was actually done during a build (i.e. it is false for no-change builds). Then for 'manifest' commands, it suppresses some console output if the build was unchanged.

This means that after the first time you run a cargo script and it's all cached, you only see output from the actual command; nothing from cargo (assuming your code has no warnings).

Fixes #16388

Testing

1. Create this script (if I add any dependencies it gives me grief about mismatched rustc versions or something but we don't need any to test this fortunately):

#!/usr/bin/env -S /path/to/cargo/target/debug/cargo -Zscript
---
[package]
edition = "2024"
[dependencies]
---

fn main() {
    println!("Hello");
}

2. Build cargo with nightly:

cargo +nightly build

3. Run the script once. You should see some compilation output messages from Cargo.
4. Run it again. This time you should only see Hello.

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[Timmmm]

Note I wasn't exactly sure what the significance of Exec::Manifest is. I couldn't see any comments and it seems to be used for cargo script and some other stuff, but I'm not sure exactly what. Also I don't know what that xtask-bump-check is but I guess this probably doesn't affect it.

[epage]

Note that our contrib docs discourage PRs until the issue has been accepted by the Cargo team

Going to close for now until something is decided. We can always re-open if this PR is aligned with the agreed-to direction.