Rustc: Simplify --sysroot code after rust#103660

[xFrednet]

If I'm honest, I still don't fully understand when and where the --sysroot argument is required. Clippy has very few comments on the topic. Miri has a few more, but also a custom setup in their cargo-miri interface, which is probably needed for the way miri operates.

Before rust-lang/rust#103660, rustc used to use the path of the executable to get the system root, that's why Clippy and Miri had custom ways to search for the system root. When I wrote Marker's driver, I took Clippy's code for this part and refactored it for readability.

The referenced PR changed rustc's default behavior to use the path of the rustc library, which should work for Clippy, Miri and Marker. Clippy and Miri still have some minimal code, to check for an environment value. This PR mimics Clippy's behavior, with the deviation that we don't use SYSROOT, but MARKER_SYSROOT as the name of the environment value. I think this is a bit cleaner and inline with Miri, which uses MIRI_SYSROOT

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Closes #48

Maybe solves #324 as well? I sadly don't have a setup to replicate the reported bug. I'm considering, if we maybe want to do a test release and ask @tigerros (the author of #324) if they can test the release binaries for us. This PR has the v0.4.2 tag as the baseline, so we can just create a PR to the hotfix/0.4 branch for that.

@Veetaha would you mind taking a look at this PR?

[Veetaha]

I wouldn't worry about copying clippy's behavior. The less custom setup there is without breaking anything (at least in tests) the better. It's was likely some legacy code

[xFrednet]

Yep agreed. Do you think we should do a patch version for this? I'm a little scared, that it might break things. But I'm guessing to 99% that it will be fine, and it might even fix the bug

[Veetaha]

Let's try to do a patch 0.4.3-rc. I remember there was some caveat with this with the toolchain version state during prereleases. But since the toolchain version is currently the same as in the patch there shouldn't be any problem? Anyhow, even if the released version that uses an older toolchain and we use the hotfix branch with that toolchain in it it shouldn't cause any problems I think, but I may be misremembering