Store vDSO elf in the cache dir

Test Plan
=========

```
$ ls /tmp/lightswitch
dumped-vdso  unwind-info
```

src/profiler.rs

@@ -11,7 +11,7 @@ use std::mem::ManuallyDrop;
 use std::mem::MaybeUninit;
 use std::os::fd::{AsFd, AsRawFd};
 use std::os::unix::fs::FileExt;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use std::thread;
 use std::time::{Duration, Instant};
@@ -82,6 +82,7 @@ impl NativeUnwindState {
 }
 
 pub struct Profiler {
+    cache_dir: PathBuf,
     // Prevent the links from being removed.
     _links: Vec<Link>,
     native_unwinder_open_object: ManuallyDrop<Box<MaybeUninit<OpenObject>>>,
@@ -193,6 +194,7 @@ fn fetch_vdso_info(
     start_addr: u64,
     end_addr: u64,
     offset: u64,
+    cache_dir: &Path,
 ) -> Result<(PathBuf, ObjectFile)> {
     // Read raw memory
     let file = fs::File::open(format!("/proc/{}/mem", pid))?;
@@ -201,7 +203,7 @@ fn fetch_vdso_info(
     file.read_exact_at(&mut buf, start_addr + offset)?;
 
     // Write to a temporary place
-    let dumped_vdso = PathBuf::from("/tmp/lightswitch-dumped-vdso");
+    let dumped_vdso = cache_dir.join("dumped-vdso");
     fs::write(&dumped_vdso, &buf)?;
 
     // Pass that to the object parser
@@ -387,6 +389,7 @@ impl Profiler {
         let profile_receive = Arc::new(receiver);
 
         Profiler {
+            cache_dir: profiler_config.cache_dir,
             _links: Vec::new(),
             native_unwinder_open_object,
             native_unwinder,
@@ -1674,9 +1677,13 @@ impl Profiler {
                     // be careful, the kernel might be upgraded since last time we ran, and that cache might not be
                     // valid anymore.
 
-                    if let Ok((vdso_path, object_file)) =
-                        fetch_vdso_info(pid, map.address.0, map.address.1, map.offset)
-                    {
+                    if let Ok((vdso_path, object_file)) = fetch_vdso_info(
+                        pid,
+                        map.address.0,
+                        map.address.1,
+                        map.offset,
+                        &self.cache_dir,
+                    ) {
                         let mut object_files = object_files_clone.write();
                         let Ok(executable_id) = object_file.id() else {
                             debug!("vDSO object file id failed");