Set WASMTIME_BACKTRACE_DETAILS=1 for WASM basictest

https://github.com/ruby/ruby/actions/runs/15008767265/job/42173424631
"error while executing at wasm backtrace" doesn't have meaningful
symbols. We can't debug this from just looking at the "[BUG] Cannot
malloc during GC" message.

As suggested by the error message, this commit sets WASMTIME_BACKTRACE_DETAILS=1.
Let me see if this improves the backtrace.

Author: k0kubun

.github/workflows/wasm.yml

@@ -152,6 +152,8 @@ jobs:
 
       - name: Run basictest
         run: wasmtime run ./../build/miniruby --mapdir /::./ -- basictest/test.rb
+        env:
+          WASMTIME_BACKTRACE_DETAILS: '1'
         working-directory: src
 
       - name: Run bootstraptest (no thread)