fix: Unify `root` and non-root user behavior

[runiq]

This makes building as root and as a non-root user behave the same.

Before, building as a non-root user (i.e., via podman run --userns=keep-id) would be different in the following ways:

• It would not automatically source /etc/profile.d/cargo.sh (at least for me)
• As a result, RUSTUP_HOME would not be set and cargo would complain about not having a default target
• It would not build against the x86_64-unknown-linux-musl target because cargo's default config location is ~/.cargo/config, but we only write our desired build target into ~root/.cargo/config, which is not checked when a non-root user invokes cargo

This PR fixes all that by configuring all those settings as environment variables instead. It also sets the cargo PATH via an environment variable, so /etc/profile.d/cargo.sh is not needed anymore.

A downside of this approach is that the build target triple cannot be overridden via config files because the CARGO_BUILD_TARGET environment variable has higher precedence than a ~/.cargo/config entry. I figured this would be fine since the entire point of this project is to build against that specific target triple. :upside_down_face:

[clux]

Hey, thanks for this. Yeah, I think this is a slightly better setup as well.

A downside of this approach is that the build target triple cannot be overridden via config files because the CARGO_BUILD_TARGET environment variable has higher precedence than a ~/.cargo/config entry. I figured this would be fine since the entire point of this project is to build against that specific target triple.

Yeah, I would be surprised if there are anyone overriding it in this image, but we'll see. At any rate, I think it's ok to make this change.