Support rustflags from cargo config

[paholg]

Cargo will ignore any rustflags set in a config file if RUSTFLAGS is
set. Because cargo-fuzz always sets RUSTFLAGS, flags in config files
are always ignored.

So, instead of just pulling prior flags from RUSTFLAGS, we can rely on
cargo to find them for us.

[paholg]

The build failure appears to be a transient network error.

[fitzgen]

I really don't want to pull in cargo as a library. It's a ton of dev overhead; really slow to compile and we'll be chasing updates every 6 weeks. I talked with @alexcrichton and he confirmed that using cargo as a library is discouraged, and that instead we should file an issue on the cargo repo to add a CLI flag to cargo to print out the information we need, and then we can just spawn a subprocess to get this information.

Can you file a bug upstream requesting a CLI flag to read the rustflags from the cargo config, and then rewrite this PR to use a cargo sub-process? Thanks!

[paholg]

Fair enough, will do.

[paholg]

It turns out this was just recently implemented in cargo via cargo config, so I just need to wait for it to stabilize.

rust-lang/cargo#9302