We should try to deduplicate dep trees between benchmarches.

A global build cache (to avoid a common workspace) would be great but probably hard.

The next best thing is likely allowing a single workspace to have multiple benchmarks, and using a single target directory for building their dependencies (e.g. the Servo crates might benefit from this).

We can also try to reuse -debug artifacts for -check builds, further reducing times (although that might be harder, since --emit=metadata produces rmetas different from --emit=metadata,link, and cargo check likely still can't use regular artifacts).

Start by examining how benchmarks currently obtain separate workspaces and dependency target directories; the issue names no files or tests. Compare the proposed shared workspace and target directory approach with reuse of -debug artifacts for -check builds. Done means benchmark dependency trees are deduplicated without invalid artifact reuse.