feat(tvix/store): add new_temporary for all Sled services

This provides a service using /dev/shm, that's deleted once the
reference is dropped.

Refactor all tests to use these, which allows getting rid of most
TempDir usage in the tests.

The only place where we still use TempDir is in the importer tests,
which work on a filesystem path.

Change-Id: I08a950aa774bf9b46d9f5c92edf5efba36053242
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8193
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/src/blobservice/sled.rs

@@ -20,6 +20,13 @@ impl SledBlobService {
 
         Ok(Self { db })
     }
+
+    pub fn new_temporary() -> Result<Self, sled::Error> {
+        let config = sled::Config::default().temporary(true);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
 }
 
 impl BlobService for SledBlobService {