Use ctime for interpreter timestamps (#1067)

Per https://apenwarr.ca/log/20181113, `ctime` should be a lot more
conservative, and should detect things like the issue we see with the
python-build-standalone builds, where the `mtime` is identical across
builds.

On Windows, I'm just using `last_write_time`. But we should probably add
`volume_serial_number` and other attributes via
[`winapi_util`](https://docs.rs/winapi-util/latest/winapi_util/index.html).

crates/puffin-cache/src/by_timestamp.rs

@@ -1,9 +1,7 @@
-use std::time::SystemTime;
-
 use serde::{Deserialize, Serialize};
 
 #[derive(Deserialize, Serialize)]
-pub struct CachedByTimestamp<T> {
-    pub timestamp: SystemTime,
-    pub data: T,
+pub struct CachedByTimestamp<Timestamp, Data> {
+    pub timestamp: Timestamp,
+    pub data: Data,
 }