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

This allows each update for a pull request to reuse the ccache data
from the previous run for that pull request.  This will help improve
build times since the ccache data used will be specific to the
pull request and not polluted with builds of other PRs.

Author: tstellar

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

@@ -0,0 +1,25 @@
+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/
+        unzip ${{ steps.download-artifact.outputs.filename }}
+        rm ${{ steps.download-artifact.outputs.filename }}
+        tar --zstd -xf ccache.tar.zst
+        rm ccache.tar.zst
+        ls -altr

.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 +27,10 @@ jobs:
         uses: hendrikmuhs/[email protected]
         with:
           max-size: "2000M"
+      - 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 +74,9 @@ 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