workflows/premerge: Generate a ccache artifact for each pull request

.github/workflows/pr-ccache-restore/action.yml

@@ -0,0 +1,23 @@
+name: PR ccache restore
+
+inputs:
+  artifact-name-suffix:
+    desciption: The suffix to append to the artifict name (ccache-pr#)
+    required: true
+
+runs:
+  using: "composite"
+  steps:
+    - uses: ./.github/workflows/unprivileged-download-artifact
+      id: download-artifact
+      with:
+        artifact-name: ccache-pr${{ github.event.pull_request.number }}-${{ inputs.artifact-name-suffix }}
+
+    - shell: bash
+      if: steps.download-artifact.outputs.filename != ''
+      run: |
+        # Is this the best way to clear the cache?
+        rm -Rf .ccache/
+        rm ${{ steps.download-artifact.outputs.filename }}
+        tar --zstd -xf ccache.tar.zst
+        rm ccache.tar.zst

.github/workflows/pr-ccache-save/action.yml

@@ -0,0 +1,25 @@
+name: PR ccache save
+inputs:
+  artifact-name-suffix:
+    desciption: The suffix to append to the artifict name (ccache-pr#)
+    required: true
+
+runs:
+  using: "composite"
+  steps:
+    - name: Package ccache Directory
+      shell: bash
+      run: |
+        # Dereference symlinks so that this works on Windows.
+        tar -h -c .ccache | zstd -T0 -c > ccache.tar.zst
+    - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 #v4.3.0
+      with:
+        name: 'ccache-pr${{ github.event.number }}-${{ inputs.artifact-name-suffix }}'
+        path: ccache.tar.zst
+        retention-days: 7
+        overwrite: true
+
+    - shell: bash
+      run: |
+        rm ccache.tar.zst
+        ccache --show-stats

.github/workflows/premerge.yaml

@@ -27,6 +27,13 @@ jobs:
         uses: hendrikmuhs/[email protected]
         with:
           max-size: "2000M"
+      - name: Install dependencies for pr-ccache-restore
+        run: |
+          sudo apt-get install zstd unzip
+      - name: Restore ccache from previous PR run
+        uses: ./.github/workflows/pr-ccache-restore
+        with:
+          artifact-name-suffix: Linux-x86
       - name: Build and Test
         # Mark the job as a success even if the step fails so that people do
         # not get notified while the new premerge pipeline is in an
@@ -70,3 +77,9 @@ jobs:
           export CXX=/opt/llvm/bin/clang++
 
           ./.ci/monolithic-linux.sh "$(echo ${linux_projects} | tr ' ' ';')" "$(echo ${linux_check_targets})" "$(echo ${linux_runtimes} | tr ' ' ';')" "$(echo ${linux_runtime_check_targets})"
+
+      - name: Save ccache for next PR run
+        if: always()
+        uses: ./.github/workflows/pr-ccache-save
+        with:
+          artifact-name-suffix: Linux-x86