Build MLIR and CIRCT cache for master

The cache is not shared between branches resulting in CIRCT and
MLIR being built each time a new PR is created.

This created a cached version of MLIR and CIRCT for master, which
is shared with derived branches.

.github/workflows/DefaultCache.yml

@@ -0,0 +1,19 @@
+name: DefaultCache
+
+on:
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  DefaultCache:
+    runs-on: ubuntu-22.04
+    steps:
+    - name: "Install dependencies"
+      uses: ./.github/actions/InstallPackages
+      with:
+        install-llvm: true
+        install-clang: true
+    - name: "Build CIRCT to generate cache"
+      uses: ./.github/actions/BuildCirct
+    - name: "Build MLIR to generate cache"
+      uses: ./.github/actions/BuildMlirDialect