Use distinct ccaches

Since they run in distinct jobs, using the same ccache would
cause one job to overwrite the cache of the other.

See https://github.com/ljfitz/torch-mlir/pull/16 for a proof
that this works. The first build takes a long time but ccache
takes over in the dummy commit.

.github/actions/setup-build/action.yml

@@ -1,6 +1,14 @@
 name: "Setup build environment"
 description: "Setup the build environment. An action so that it can be shared between in-tree/out-of-tree jobs"
 
+inputs:
+  cache-key:
+    description: |
+      Additional string that is used to compute the ccache hash.
+      Different jobs running the action need distinct values for this key,
+      but the content is irrelevant.
+    required: true
+
 runs:
   using: "composite"
   steps:
@@ -25,4 +33,4 @@ runs:
   - name: Ccache for C++ compilation
     uses: hendrikmuhs/ccache-action@4687d037e4d7cf725512d9b819137a3af34d39b3
     with:
-      key: ${{ runner.os }}-clangreleaseasserts-${{ steps.get-submodule-hash.outputs.hash }}
+      key: ${{ runner.os }}-clangreleaseasserts-${{ steps.get-submodule-hash.outputs.hash }}-${{ inputs.cache-key }}

.github/workflows/buildAndTest.yml

@@ -24,6 +24,8 @@ jobs:
       with:
         submodules: 'true'
     - uses: ./.github/actions/setup-build
+      with:
+        cache-key: 'in-tree'
     - name: Build and Test torch-mlir (Assert)
       run: |
         cd $GITHUB_WORKSPACE
@@ -97,6 +99,8 @@ jobs:
       with:
         submodules: 'true'
     - uses: ./.github/actions/setup-build
+      with:
+        cache-key: 'out-of-tree'
     - name: Build LLVM (standalone)
       run: |
         cd $GITHUB_WORKSPACE