opt-dist disk usage broken

Our disk usage calculation in opt-dist (https://github.com/rust-lang/rust/blob/8a3308ed506be56b8157dfc9242a145153331296/src/tools/opt-dist/src/utils/mod.rs#L18) assumes that distinct disks will not be reported twice. However, currently sysinfo will report docker mounts (overlay fs) in addition to the main disk, leading to buggy double-counting:

$ docker run --rm -it -v `pwd`:/mnt ubuntu:26.04 /mnt/target/debug/check-sysinfo
Emulate Docker CLI using podman. Create /etc/containers/nodocker to quiet msg.
[src/main.rs:3:9] disk = Disk("/dev/root")[FS: "ext4"][Type: Unknown(-1)][removable: no][I/O: DiskUsage { total_written_bytes: 0, written_bytes: 0, total_read_bytes: 0, read_bytes: 0 }] mounted on "/mnt": 225873375232/311227166720 B
[src/main.rs:4:9] humansize::format_size(disk.available_space(), humansize::BINARY) = "210.36 GiB"
[src/main.rs:3:9] disk = Disk("overlay")[FS: "overlay"][Type: Unknown(-1)][removable: no][I/O: DiskUsage { total_written_bytes: 0, written_bytes: 0, total_read_bytes: 0, read_bytes: 0 }] mounted on "/": 225873375232/311227166720 B
[src/main.rs:4:9] humansize::format_size(disk.available_space(), humansize::BINARY) = "210.36 GiB"

I suspect this is also the root cause of why Crater keeps filing up its disks... cc https://github.com/rust-lang/crater/issues/762

Start at src/tools/opt-dist/src/utils/mod.rs#L18 and reproduce the duplicate disk reporting with the Docker command shown in the issue. Inspect how opt-dist consumes sysinfo disk entries; done means disk usage is no longer double-counted when overlay and main-disk mounts report the same space.