rustjail: Fix clippy error

```
error: file opened with `create`, but `truncate` behavior not defined
```
`truncate(true)` ensures the file is entirely overwritten with new data
which I believe is the behaviour we want

Signed-off-by: stevenhorsman <steven@uk.ibm.com>

src/agent/rustjail/src/mount.rs

@@ -836,6 +836,7 @@ fn mount_from(
         if !src.is_dir() {
             let _ = OpenOptions::new()
                 .create(true)
+                .truncate(true)
                 .write(true)
                 .open(&dest)
                 .map_err(|e| {