Set --vanilla to prevent cli's task callback

This task callback was causing intermittent unexpected output:

https://github.com/r-lib/cli/blob/1220ed092c03e167ff0062e9839c81d7258a4600/R/onload.R#L33-L40

Note that `istty(stdout())` is `TRUE` in these tests even though it's
`FALSE` in Positron. That's likely because we don't redirect stdout, see
https://github.com/posit-dev/ark/blob/main/crates/amalthea/src/sys/unix/stream_capture.rs#L39-L42

For clarity and consistency with the regular path, we also set
`--interactive`

Author: lionel-

crates/ark/src/fixtures/dummy_frontend.rs

@@ -45,7 +45,12 @@ impl DummyArkFrontend {
         stdext::spawn!("dummy_kernel", || {
             crate::start::start_kernel(
                 connection_file,
-                vec![String::from("--no-save"), String::from("--no-restore")],
+                vec![
+                    String::from("--interactive"),
+                    String::from("--vanilla"),
+                    String::from("--no-save"),
+                    String::from("--no-restore"),
+                ],
                 None,
                 SessionMode::Console,
                 false,