instrument: README: fix invocation

[fw-immunant]

The invocation here was out-of-date as of #554; the command now works as intended.

[kkysen]

How'd you determine these new invocation commands? The most up-to-date commands are in pdg/src/main.rs in mod tests, as those are the ones tested in cargo test and CI.

Could we just say to look there in the README.md?

[fw-immunant]

This is basically what's in pdg.sh, but as you note also matches src/pdg/main,rs. I don't think suggesting to read the source (even with a pointer to where) is helpful for a user-facing README--we should have a shell command so people can get started quickly.

[kkysen]

I think this still might be broken, which is why I hadn't approved it earlier. The ${CARGO_TARGET_DIR:-../target/debug/c2rust-instrument} assumes the cwd is dynamic_instrumentation/ (I think, or at least some other directory), but analysis/tests/misc/Cargo.toml is relative to the repo root. Also, I don't remember if --metadata is supposed to be relative to the cwd or to the manifest dir.

[fw-immunant]

I pushed a fix that works correctly with the repo root as cwd. Even when building the entire repo with cargo build in the repo root, c2rust-instrument ends up in the debug subdir of the cargo target directory; the other commands already expected to be there.

--metadata is relative to the cwd, which does mean that we need it to include the base path of the Cargo.toml.