fix: avoid deadlock in nested shell calls

[DaniPopes]

cc @zerosnacks #9244 (comment)

[zerosnacks]

Thanks @DaniPopes!

Confirming this works on nested shell calls of arbitrary depth

        sh_println!("{:?}", {
            sh_println!("Outer")?;
            sh_println!("{:?}", {
                sh_println!("Middle")?;
                sh_println!("{:?}", {
                    sh_println!("Inner")?;
                    "deepest nested"
                })?;
                "nested middle"
            })?;
            "nested outer"
        })?;