Fix syntax

Author: ldionne

.github/workflows/libcxx-build-and-test.yaml

@@ -50,7 +50,7 @@ jobs:
   stage1:
     if: github.repository_owner == 'llvm'
     runs-on: libcxx-self-hosted-linux
-    container: ghcr.io/llvm/libcxx-linux-builder:${{ LIBCXX_LINUX_IMAGE_TAG }}
+    container: ghcr.io/llvm/libcxx-linux-builder:${{ env.LIBCXX_LINUX_IMAGE_TAG }}
     continue-on-error: false
     strategy:
       fail-fast: false
@@ -87,7 +87,7 @@ jobs:
   stage2:
     if: github.repository_owner == 'llvm'
     runs-on: libcxx-self-hosted-linux
-    container: ghcr.io/llvm/libcxx-linux-builder:${{ LIBCXX_LINUX_IMAGE_TAG }}
+    container: ghcr.io/llvm/libcxx-linux-builder:${{ env.LIBCXX_LINUX_IMAGE_TAG }}
     needs: [ stage1 ]
     continue-on-error: false
     strategy:
@@ -175,7 +175,7 @@ jobs:
         - config: 'generic-msan'
           machine: libcxx-self-hosted-linux
     runs-on: ${{ matrix.machine }}
-    container: ghcr.io/llvm/libcxx-linux-builder:${{ LIBCXX_LINUX_IMAGE_TAG }}
+    container: ghcr.io/llvm/libcxx-linux-builder:${{ env.LIBCXX_LINUX_IMAGE_TAG }}
     steps:
       - uses: actions/checkout@v4
       - name: ${{ matrix.config }}