Use ninja to build llvm and circt, pin Ubuntu version in CI

Uing makefiles and parallel build seems to cause errors in CI.

.github/build-circt.sh

@@ -3,10 +3,11 @@ set -e
 
 mkdir -p circt/build
 cd circt/build
-cmake .. \
+cmake -G Ninja .. \
     -DCMAKE_BUILD_TYPE=Release \
     -DMLIR_DIR=$PWD/../llvm/build/lib/cmake/mlir \
     -DLLVM_DIR=$PWD/../llvm/build/lib/cmake/llvm \
     -DLLVM_ENABLE_ASSERTIONS=ON
 
-cmake --build . -- -j$(nproc)
+ninja -j$(nproc)
+

.github/build-llvm.sh

@@ -3,7 +3,7 @@ set -e
 
 mkdir -p circt/llvm/build
 cd circt/llvm/build
-cmake ../llvm \
+cmake -G Ninja ../llvm \
     -DCMAKE_BUILD_TYPE=Release \
     -DLLVM_BUILD_EXAMPLES=OFF \
     -DLLVM_ENABLE_ASSERTIONS=ON \
@@ -14,4 +14,4 @@ cmake ../llvm \
     -DLLVM_OPTIMIZED_TABLEGEN=ON \
     -DLLVM_TARGETS_TO_BUILD="host"
 
-cmake --build . -- -j$(nproc)
+ninja -j$(nproc)

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ name: CI
 jobs:
   lints:
     name: Lints
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout sources
         uses: actions/checkout@v2
@@ -32,7 +32,7 @@ jobs:
 
   check:
     name: Checks
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout sources
         uses: actions/checkout@v2
@@ -64,7 +64,9 @@ jobs:
 
       - name: Rebuild LLVM
         if: steps.cache-llvm.outputs.cache-hit != 'true'
-        run: .github/build-llvm.sh
+        run: |
+          sudo apt-get install ninja-build
+          .github/build-llvm.sh
 
       - name: Upload LLVM to cache
         if: steps.cache-llvm.outputs.cache-hit != 'true'
@@ -83,7 +85,9 @@ jobs:
 
       - name: Rebuild CIRCT
         if: steps.cache-circt.outputs.cache-hit != 'true'
-        run: .github/build-circt.sh
+        run: |
+          sudo apt-get install ninja-build
+          .github/build-circt.sh
 
       - name: Upload CIRCT to cache
         if: steps.cache-circt.outputs.cache-hit != 'true'