GHA: use macos-11 runner

According to the following github document,
macos-latest is either macos-11 or macos-12 at this point.

https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

It's better for us to use a consistent version to build.

This might explain the following error i've seen with wasi-sdk-17.0
release binary on macos-11.0:

```
    Run Build Command(s):/usr/bin/make -f Makefile cmTC_254ef/fast && /Applications/Xcode_13.2.1.app/Contents/Developer/usr/bin/make  -f CMakeFiles/cmTC_254ef.dir/build.make CMakeFiles/cmTC_254ef.dir/build
    Building C object CMakeFiles/cmTC_254ef.dir/testCCompiler.c.obj
    /Users/runner/work/toywasm/toywasm/.wasi-sdk-17.0/bin/clang    -o CMakeFiles/cmTC_254ef.dir/testCCompiler.c.obj -c /Users/runner/work/toywasm/toywasm/build.wasm/CMakeFiles/CMakeScratch/TryCompile-aWXHXM/testCCompiler.c
    dyld: Symbol not found: __ZTTNSt3__114basic_ifstreamIcNS_11char_traitsIcEEEE
      Referenced from: /Users/runner/work/toywasm/toywasm/.wasi-sdk-17.0/bin/clang (which was built for Mac OS X 12.0)
      Expected in: /usr/lib/libc++.1.dylib
     in /Users/runner/work/toywasm/toywasm/.wasi-sdk-17.0/bin/clang
    make[1]: *** [CMakeFiles/cmTC_254ef.dir/testCCompiler.c.obj] Abort trap: 6
    make: *** [cmTC_254ef/fast] Error 2
```

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
       matrix:
         os:
           - ubuntu-20.04
-          - macos-latest
+          - macos-11
     steps:
       - uses: actions/cache@v3
         with:
@@ -34,10 +34,10 @@ jobs:
       - uses: actions/cache@v3
         with:
           path: ~/Library/Caches/ccache
-          key: 0-cache-macos-latest-${{ github.run_id }}
+          key: 0-cache-${{ matrix.os }}-${{ github.run_id }}
           restore-keys: |
-            0-cache-macos-latest
-        if: matrix.os == 'macos-latest'
+            0-cache-${{ matrix.os }}
+        if: startsWith(matrix.os, 'macos-')
       - name: Install wasmtime for tests
         run: curl -f -L --retry 5 https://wasmtime.dev/install.sh | bash -s -- --version v2.0.2
       - uses: actions/checkout@v3
@@ -51,7 +51,7 @@ jobs:
       - run: git submodule update --init --depth 16 --jobs 3
       - name: Install ccache, ninja (macOS)
         run: brew install ccache ninja
-        if: matrix.os == 'macos-latest'
+        if: startsWith(matrix.os, 'macos-')
       - name: Install ccache, ninja (Linux)
         run: sudo apt install ccache ninja-build
         if: startsWith(matrix.os, 'ubuntu-')