process: fix the process_kill_on_drop.rs test on non-Linux systems (#3203)

"disown" is a bash builtin, not part of POSIX sh.

.cirrus.yml

@@ -11,7 +11,7 @@ task:
     LOOM_MAX_PREEMPTIONS: 2
     RUSTFLAGS: -Dwarnings
   setup_script:
-    - pkg install -y curl
+    - pkg install -y bash curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh
     - sh rustup.sh -y --profile minimal --default-toolchain stable
     - . $HOME/.cargo/env

tokio/tests/process_kill_on_drop.rs

@@ -10,7 +10,7 @@ use tokio_test::assert_ok;
 
 #[tokio::test]
 async fn kill_on_drop() {
-    let mut cmd = Command::new("sh");
+    let mut cmd = Command::new("bash");
     cmd.args(&[
         "-c",
         "