Linux: ensure non-interactive install

apt needs -y and it also needs DEBIAN_FRONTEND=noninteractive
to avoid questions, from a root shell.

Closes #117.

src/linux.rs

@@ -175,6 +175,7 @@ fn add_deb(path: &OsStr) -> Result<(), Box<dyn Error>> {
     let mut args: Vec<OsString> = vec![];
     args.push(os("install"));
     args.push(os("--reinstall"));
+    args.push(os("-y"));
     // https://askubuntu.com/a/668859
     args.push(os("-o=Dpkg::Use-Pty=0"));
     args.push(path.to_os_string());

src/run.rs

@@ -26,6 +26,7 @@ pub fn run(cmd: OsString, args: Vec<OsString>, _what: &str)
 
     debug!("Running {:?} with args {:?}", cmd, args);
     let reader = duct::cmd(cmd, args)
+	.env("DEBIAN_FRONTEND", "noninteractive")
         .stderr_to_stdout()
         .reader()?;
     let lines = BufReader::new(reader).lines();