ci: Move iverilog from build to test

.github/workflows/test-linux.yml

@@ -104,30 +104,6 @@ jobs:
         uses: actions/checkout@v4
         with:
          submodules: true
-      - name: Get iverilog
-        shell: bash
-        run: |
-          git clone https://github.com/steveicarus/iverilog.git
-          cd iverilog
-          echo "IVERILOG_GIT=$(git rev-parse HEAD)" >> $GITHUB_ENV
-
-      - name: Cache iverilog
-        id: cache-iverilog
-        uses: actions/cache@v4
-        with:
-          path: .local/
-          key: ${{ matrix.os.id }}-${{ env.IVERILOG_GIT }}
-
-      - name: Build iverilog
-        if: steps.cache-iverilog.outputs.cache-hit != 'true'
-        shell: bash
-        run: |
-          mkdir -p $GITHUB_WORKSPACE/.local/
-          cd iverilog
-          autoconf
-          CC=gcc CXX=g++ ./configure --prefix=$GITHUB_WORKSPACE/.local
-          make -j${{ env.procs }}
-          make install
 
       - name: Build yosys out-of-tree
         shell: bash
@@ -158,7 +134,11 @@ jobs:
   test-linux:
     name: Run tests
     needs: build-linux
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os.id }}
+    strategy:
+      matrix:
+        os:
+          - { id: ubuntu-20.04, name: focal }
     steps:
       - name: Install Dependencies
         shell: bash
@@ -171,11 +151,39 @@ jobs:
         env:
           WORKSPACE: ${{ github.workspace }}
         run: |
+          echo "GITHUB_WORKSPACE=`pwd`" >> $GITHUB_ENV
+          echo "$GITHUB_WORKSPACE/.local/bin" >> $GITHUB_PATH
           echo "procs=$(nproc)" >> $GITHUB_ENV
 
       - name: Checkout Yosys
         uses: actions/checkout@v4
 
+      - name: Get iverilog
+        shell: bash
+        run: |
+          git clone https://github.com/steveicarus/iverilog.git
+          cd iverilog
+          git checkout 192b6aec96fde982e6ddcb28b346d5893aa8e874
+          echo "IVERILOG_GIT=$(git rev-parse HEAD)" >> $GITHUB_ENV
+
+      - name: Cache iverilog
+        id: cache-iverilog
+        uses: actions/cache@v4
+        with:
+          path: .local/
+          key: ${{ matrix.os.id }}-${{ env.IVERILOG_GIT }}
+
+      - name: Build iverilog
+        if: steps.cache-iverilog.outputs.cache-hit != 'true'
+        shell: bash
+        run: |
+          mkdir -p $GITHUB_WORKSPACE/.local/
+          cd iverilog
+          autoconf
+          CC=gcc CXX=g++ ./configure --prefix=$GITHUB_WORKSPACE/.local
+          make -j${{ env.procs }}
+          make install
+
       - name: Download build artifact
         uses: actions/download-artifact@v4
         with: