Explicitly passing --sysroot should no longer be necessary

[bjorn3]

Rustc has been able to find the sysroot based on the location of librustc_driver.so in addition to the location of the main executable for a while now.

[kkysen]

We're still on a very old version of rustc, though (from August 2022 I think). Do you know what version this was fixed in?

[bjorn3]

It was fixed November 5th of 2022: rust-lang/rust#103660, so just barely too late for c2rust I guess.

[kkysen]

Ahh, hopefully we'll be able to upgrade a little bit more at some point. There are a lot of other useful things we're missing, too.