Fix CI to use install-llvm-action CC/CXX instead of clang-21.

The cached LLVM install exposes clang/clang++ on PATH via env: true, not
version-suffixed binary names.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: jameslavrenz

.github/workflows/ci.yml

@@ -20,10 +20,8 @@ jobs:
         with:
           version: "21"
           cached: true
+          env: true
       - name: Build and test (MCU)
-        env:
-          CXX: clang++-21
-          CC: clang-21
         run: |
           make all
           make test_c_api_smoke
@@ -37,10 +35,8 @@ jobs:
         with:
           version: "21"
           cached: true
+          env: true
       - name: Build and test (MPU)
-        env:
-          CXX: clang++-21
-          CC: clang-21
         run: make mpu
 
   mpu-asan-ubuntu:
@@ -52,10 +48,8 @@ jobs:
         with:
           version: "21"
           cached: true
+          env: true
       - name: Build and test (MPU + ASan/UBSan)
-        env:
-          CXX: clang++-21
-          CC: clang-21
         run: |
           make clean
           make mpu \
@@ -72,8 +66,9 @@ jobs:
         with:
           version: "21"
           cached: true
+          env: true
       - name: Configure
-        run: cmake -B build -DMEMKIT_EMBEDDED_LINUX=ON -DCMAKE_CXX_COMPILER=clang++-21 -DCMAKE_C_COMPILER=clang-21
+        run: cmake -B build -DMEMKIT_EMBEDDED_LINUX=ON -DCMAKE_CXX_COMPILER="$CXX" -DCMAKE_C_COMPILER="$CC"
       - name: Build
         run: cmake --build build
       - name: Test