Use session close function from oxide fork of yubihsm.rs.

The `Client` type previously had a `Drop` implementation that closed the
session if the Client had an open one. This seems to have caused
problems in other downstream projects and was subsequently removed:
iqlusioninc/tmkms#37
iqlusioninc/yubihsm.rs#265

The replacement was to provide a `session()` function that returns an
Arc / MutexGuard wrapped reference to the optional session. This isn't
useful for us here because we don't and AFAIK can't take ownership of
the session which we need because the Sesison::close function consumes
the session (it can't be reopened). Our solution requires an upstream
change to the `Client` type adding a `close_session` function that just
closes the session if one is open.

Cargo.toml

@@ -33,5 +33,5 @@ thiserror = "1.0.58"
 # vsss-rs v3 has a dependency that requires rustc 1.65 but we're pinned
 # to 1.64 till offline-keystore-os supports it
 vsss-rs = "2.7.1"
-yubihsm = { version = "0.42.1", features = ["usb", "untested"] }
+yubihsm = { git = "https://github.com/oxidecomputer/yubihsm.rs", branch = "session-close", features = ["usb", "untested"] }
 zeroize = "1.7.0"

src/ca.rs

@@ -492,6 +492,7 @@ pub fn sign(
                 }
                 return Err(e);
             }
+            hsm.client.close();
         } else {
             error!("Unknown input spec: {}", path.display());
         }