[ci] Downgrade LLVM 14 to 12 (#29)

Author: swift-kim

.github/workflows/build.yml

@@ -53,7 +53,7 @@ jobs:
         run: |
           src/tools/gn \
             --target-cpu ${{ matrix.arch }} \
-            --target-toolchain /usr/lib/llvm-14 \
+            --target-toolchain /usr/lib/llvm-12 \
             --target-dir build
           ninja -C src/out/build
 
@@ -62,7 +62,7 @@ jobs:
         run: |
           src/tools/gn \
             --target-cpu ${{ matrix.arch }} \
-            --target-toolchain /usr/lib/llvm-14 \
+            --target-toolchain /usr/lib/llvm-12 \
             --target-sysroot src/sysroot-6.5/${{ matrix.arch }} \
             --api-version 6.5 --system-cxx \
             --target-dir build