verification: `cargo verify` doesn't propagate `cargo build` error codes

[stefano-garzarella]

cargo verify on current main (697e6c5) fails but it returns 0, so the CI is not failing.

It seems that cargo verify doesn't propagate cargo build error status:

$ cargo verify; echo "\ncargo verify - exit code: $?" 
RUSTC="verus-rustc" RUSTUP_TOOLCHAIN="1.91.0" VERUS="/home/stefano/.cargo/bin/verus" VERUS_TARGETS="svsm,verify_external,verify_proof,verus_stub,vstd" "cargo" "build" "--features" "verus"
   Compiling svsm v0.1.0 (/home/stefano/repos/coconut/svsm/kernel)
error[E0425]: cannot find value `item_size` in this scope
    --> kernel/src/mm/alloc.rs:1034:28
     |
1034 |                 item_size: item_size as u64,
     |                            ^^^^^^^^^ not found in this scope

For more information about this error, try `rustc --explain E0425`.
error: could not compile `svsm` (lib) due to 1 previous error
cargo build failed with status: exit status: 101

cargo verify - exit code: 0

@ziqiaozhou @Chris-Hawblitzel I'm not sure where to open the issue, so for now I'm opening it here.