Auto merge of #6720 - ehuss:build-script-fingerprint, r=alexcrichton

Include build script execution in the fingerprint.

This adds information about the execution of a build script to the fingerprint. Previously, no information was included, and cargo relied on dirty propagation in `JobQueue` to trigger recompiles. However, if two separate targets are built via separate commands (such as `cargo build` then `cargo test`), the second command did not know that the build script was updated, and thus was incorrectly treated as "fresh".

This works by including the timestamp of the last time the build script was ran in the fingerprint. For overridden build scripts, it includes the replaced output.

Fixes #4979