Note: this was renamed from --out-dir to --artifact-dir in #13809

Original issue: #4875
Implementation PR: #5203
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#artifact-dir
Issues: Z-out-dir Nightly: --out-dir

Summary
Adds --artifact-dir=PATH flag to cargo build to specify a directory to place final artifacts.

Tasks:

• cargo build --out-dir #5203
• Rename --out-dir to --artifact-dir #13809
• Remove --out-dir

Unresolved questions

• Multi-target builds (--target <one> --target <two>)
• MSVC examples cannot be exported with out-dir #7493
• Is --out-dir justified or can we find another way of accomplishing it? #6100 — concern about similarity with --target-dir, proliferation of flags
• Duplicate artifact tracking issue. #6313 — concern about name collisions in workspaces or with examples
• What exactly is the use case for this? In what scenario would it be used where you don't already have an outer build system or script that can copy the files into the correct location?
• Confusion with rustc's --out-dir (Rename --out-dir to --artifact-dir #13809)
• Discrepancy or confusion with build.rss OUT_DIR (depends on if build.rs is generating intermediate or final artifacts) (Rename --out-dir to --artifact-dir #13809)