Ability to disable cache after it's been configured on Config (#5542)

* Wasmtime: Add `Config::disable_cache`

* bench-api: Always disable the cache

* bench-api: Always get a `Config` from CLI flags

This commit fixes an issue that I ran into just now where benchmarking
one `*.so` with `--engine-flags` was giving wildly unexpected results
comparing to something without `--engine-flags`. The root cause here
appears to that when specifying `--engine-flags` the CLI parsing code is
used to create a `Config` and when omitted a `Config::new` instance is
created. The main difference between these is that for the CLI caching
is enabled by default and for `Config::new` it is not. Coupled with the
fact that caching doesn't really work for the `main` branch this ended
up giving wild results.

The fix here is to first always use the CLI parsing code to create a
`Config` to ensure that a config is consistently created. Next the
`--disable-cache` flag is unconditionally passed to the CLI parsing to
ensure that compilation actually happens.

Once applied this enables comparing an engine without flags and an
engine with flags which provides consistent results.

* Fix compile error

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

crates/wasmtime/src/config.rs

@@ -946,6 +946,23 @@ impl Config {
         Ok(self)
     }
 
+    /// Disable caching.
+    ///
+    /// Every call to [`Module::new(my_wasm)`][crate::Module::new] will
+    /// recompile `my_wasm`, even when it is unchanged.
+    ///
+    /// By default, new configs do not have caching enabled. This method is only
+    /// useful for disabling a previous cache configuration.
+    ///
+    /// This method is only available when the `cache` feature of this crate is
+    /// enabled.
+    #[cfg(feature = "cache")]
+    #[cfg_attr(nightlydoc, doc(cfg(feature = "cache")))]
+    pub fn disable_cache(&mut self) -> &mut Self {
+        self.cache_config = CacheConfig::new_cache_disabled();
+        self
+    }
+
     /// Loads cache configuration from the system default path.
     ///
     /// This commit is the same as [`Config::cache_config_load`] except that it