ci: fix Windows build failure for wasi-libc

The wasi-libc Makefile uses the `find` command line tool. Unfortunately,
it was using the Windows find version instead of the MinGW version,
leading to lots of errors at a later stage.

This commit prepends /usr/bin to `$PATH` to make sure the MinGW version
is found first.

azure-pipelines.yml

@@ -54,7 +54,7 @@ jobs:
       displayName: Build wasi-libc
       inputs:
         targetType: inline
-        script: make wasi-libc CLANG=$PWD/llvm-build/bin/clang LLVM_AR=$PWD/llvm-build/bin/llvm-ar LLVM_NM=$PWD/llvm-build/bin/llvm-nm
+        script: PATH=/usr/bin:$PATH make wasi-libc CLANG=$PWD/llvm-build/bin/clang LLVM_AR=$PWD/llvm-build/bin/llvm-ar LLVM_NM=$PWD/llvm-build/bin/llvm-nm
     - task: Bash@3
       displayName: Test TinyGo
       inputs: