propolis-package: produces output that omicron finds cromulent, despite missing propolis-server binary

I have some scripts I use which build propolis/crucible, package them as appropriate, and then generate a tuf repo with the standard tuf releng tool.

I encountered a benign compilation error with propolis, due to an out of date toolchain on my system.

error[E0658]: the `#[expect]` attribute is an experimental feature
   --> /home/vi/.cache/tuf-builder/crucible/upstairs/src/io_limits.rs:126:5
    |
126 |     #[expect(unused)]
    |     ^^^^^^^^^^^^^^^^^
    |
    = note: see issue #54503 <https://github.com/rust-lang/rust/issues/54503> for more information

Consequently, the propolis-server binary was never generated.

propolis-package did complain about this:

     Running `target/debug/propolis-package`
Error: Cannot append binary to tarfile: No such file or directory (os error 2) when getting metadata for target/release/propolis-server

However, the package output file was left in place:

vi@materia ~/.c/t/propolis (master)> tar tf out/propolis-server.tar.gz 
oxide.json
root/
root/var
root/var/svc
root/var/svc/manifest
root/var/svc/manifest/site
root/var/svc/manifest/site/propolis-server
root/var/svc/manifest/site/propolis-server/config.toml
root/var/svc/manifest/site/propolis-server/manifest.xml
root/
root/opt
root/opt/oxide
root/opt/oxide/lib
root/opt/oxide/lib/svc
root/opt/oxide/lib/svc/manifest
root/opt/oxide/lib/svc/manifest/propolis
root/opt/oxide/lib/svc/manifest/propolis/propolis.sh
root/
root/opt
root/opt/oxide
root/opt/oxide/propolis-server
root/opt/oxide/propolis-server/bin

My script proceeded to execute the tuf repo releng task, which accepted this as input. This was well up my scrollback of logs, and ultimately, I did not learn of the error until I tried to launch a VM, where found it in the "starting" state forever.

Now, truthfully, my script should have checked whether building the binary was successful and bailed before even running propolis-package. That said, in the event of errors like this, propolis-package ought to avoid placing an acceptable package at the location the tuf builder looks, perhaps by assembling the tarfile in a file with a different name, and renaming it only after it's done successfully.

Start with the propolis-package entry point and trace how it creates out/propolis-server.tar.gz when target/release/propolis-server is missing. Reproduce the missing-binary packaging error and verify that a failed run does not leave a package at the path consumed by the tuf repository releng task.