Fix running tests for non-default workspace packages

Previously, we would fail to run tests for non-default workspace
packages.

For example, given:

```toml
[workspace]
members = ["path/to/member1", "path/to/member2", "path/to/member3/*"]
default-members = ["path/to/member2", "path/to/member3/foo"]
```

We would not be able to run tests in `path/to/member1`.

This also affected workspaces with a root package, e.g.:

```toml
[workspace]
members = ["bar", "foo"]

[package]
name = "test-ws"
version = "0.1.0"
edition = "2021"
```

We would not be able to run tests in `bar` or `foo`.

Fixes #51.

lua/neotest-rust/init.lua

@@ -228,6 +228,7 @@ function adapter.build_spec(args)
         "cargo",
         "nextest",
         "run",
+        "--workspace",
         "--no-fail-fast",
         "--config-file",
         tmp_nextest_config,

tests/init_spec.lua

@@ -573,7 +573,7 @@ describe("build_spec", function()
 
             local spec = adapter.build_spec({ tree = tree })
             assert.matches(
-                "cargo nextest run %-%-no%-fail%-fast %-%-config%-file %g+ %-%-profile neotest %-%-no%-capture %-%-test%-threads 3 %-%-test test_it ",
+                "cargo nextest run %-%-workspace %-%-no%-fail%-fast %-%-config%-file %g+ %-%-profile neotest %-%-no%-capture %-%-test%-threads 3 %-%-test test_it ",
                 spec.command
             )
         end)