list-users-in-silo doesn't filter by silo

[davepacheco]

Fixes #1327.

I'd like to add a test for this in this PR.

[davepacheco]

I added a test. In my local clone, I flipped the order of these commits so that I've got the test and not the fix, and it fails with:

     Running tests/test_all.rs (target/debug/deps/test_all-0591795a455d402b)

running 1 test
test integration_tests::silos::test_silo_users_list ... FAILED

failures:

---- integration_tests::silos::test_silo_users_list stdout ----
log file: "/dangerzone/omicron_tmp/test_all-0591795a455d402b-test_silo_users_list.28038.0.log"
note: configured to log to "/dangerzone/omicron_tmp/test_all-0591795a455d402b-test_silo_users_list.28038.0.log"
thread 'integration_tests::silos::test_silo_users_list' panicked at 'assertion failed: `(left == right)`
  left: `[User { id: 001de000-05e4-4000-8000-000000004007, display_name: "privileged" }, User { id: 001de000-05e4-4000-8000-000000060001, display_name: "unprivileged" }, User { id: 6922f0b2-9a92-659b-da6b-93ad4955a3a3, display_name: "some_silo_user" }, User { id: bd75d207-37f3-4769-b808-677ae04eaf23, display_name: "can_we_see_them?" }]`,
 right: `[User { id: 6922f0b2-9a92-659b-da6b-93ad4955a3a3, display_name: "some_silo_user" }]`', nexus/tests/integration_tests/silos.rs:812:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
WARN: dropped CockroachInstance without cleaning it up first (there may still be a child process running and a temporary directory leaked)
WARN: temporary directory leaked: /dangerzone/omicron_tmp/.tmprBeqxO
WARN: dropped ClickHouseInstance without cleaning it up first (there may still be a child process running and a temporary directory leaked)


failures:
    integration_tests::silos::test_silo_users_list

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 111 filtered out; finished in 2.28s

error: test failed, to rerun pass '-p omicron-nexus --test test_all'

which is what we expect.