Don't panic if WASI_SDK_PATH not set when detecting compiler

The fedora packaging builds the wasm sysroot outside of the rust
build system. Not panicing seems consistent with the detection
logic of other targets on failure.

Author: Paul Murphy

src/bootstrap/src/utils/cc_detect.rs

@@ -221,10 +221,7 @@ fn default_compiler(
         }
 
         t if t.contains("-wasi") => {
-            let root = build
-                .wasi_sdk_path
-                .as_ref()
-                .expect("WASI_SDK_PATH mut be configured for a -wasi target");
+            let root = build.wasi_sdk_path.as_ref()?;
             let compiler = match compiler {
                 Language::C => format!("{t}-clang"),
                 Language::CPlusPlus => format!("{t}-clang++"),