mount: Use task infra to clean up error handling

We were missing the "copy child stderr to our stderr" bits
here on error.

Motivated by this bit failing with an overlayfs root.

Signed-off-by: Colin Walters <walters@verbum.org>

lib/src/mount.rs

@@ -1,7 +1,5 @@
 //! Helpers for interacting with mountpoints
 
-use std::process::Command;
-
 use anyhow::{anyhow, Context, Result};
 use camino::Utf8Path;
 use fn_error_context::context;
@@ -25,16 +23,12 @@ pub(crate) struct Findmnt {
 
 #[context("Inspecting filesystem {path}")]
 pub(crate) fn inspect_filesystem(path: &Utf8Path) -> Result<Filesystem> {
-    tracing::debug!("Inspecting {path}");
-    let o = Command::new("findmnt")
+    let desc = format!("Inspecting {path}");
+    let o = Task::new(&desc, "findmnt")
         .args(["-J", "-v", "--output-all", path.as_str()])
-        .output()?;
-    let st = o.status;
-    if !st.success() {
-        anyhow::bail!("findmnt {path} failed: {st:?}");
-    }
-    let o: Findmnt = serde_json::from_reader(std::io::Cursor::new(&o.stdout))
-        .context("Parsing findmnt output")?;
+        .quiet()
+        .read()?;
+    let o: Findmnt = serde_json::from_str(&o).context("Parsing findmnt output")?;
     o.filesystems
         .into_iter()
         .next()