Fix Github action environment version

.github/workflows/build.yml

@@ -16,7 +16,7 @@ env:
   SCCACHE_CACHE_SIZE: 200M
 jobs:
   cache-toolchains-posix:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - name: Cache toolchains (Linux, OpenWrt, Android)
@@ -44,7 +44,7 @@ jobs:
       - run: ./get-clang.sh
       - run: EXTRA_FLAGS='target_os="android"' ./get-clang.sh
   cache-toolchains-win:
-    runs-on: windows-latest
+    runs-on: windows-2019
     steps:
       - uses: actions/checkout@v2
       - name: Cache toolchains
@@ -74,7 +74,7 @@ jobs:
             unzip ninja-win.zip -d ~/bin
           fi
   cache-toolchains-mac:
-    runs-on: macos-latest
+    runs-on: macos-11
     steps:
       - uses: actions/checkout@v2
       - uses: actions/cache@v2
@@ -88,7 +88,7 @@ jobs:
       - run: ./get-clang.sh
   linux:
     needs: cache-toolchains-posix
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
@@ -151,7 +151,7 @@ jobs:
           path: src/sha256sum.txt
   android:
     needs: cache-toolchains-posix
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
@@ -221,7 +221,7 @@ jobs:
           path: src/sha256sum.txt
   win:
     needs: cache-toolchains-win
-    runs-on: windows-latest
+    runs-on: windows-2019
     strategy:
       fail-fast: false
       matrix:
@@ -286,7 +286,7 @@ jobs:
           path: src/sha256sum.txt
   mac:
     needs: cache-toolchains-mac
-    runs-on: macos-latest
+    runs-on: macos-11
     strategy:
       fail-fast: false
       matrix:
@@ -340,7 +340,7 @@ jobs:
           path: src/sha256sum.txt
   openwrt:
     needs: cache-toolchains-posix
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: