Seed Debug Roslyn compiler caches from rolling builds (#135130)

## Summary

- Add rolling-only Debug `CoreCLR_AllSubsets` builds for Linux x64,
Linux musl x64, and macOS arm64, matching the corresponding PR build
arguments.
- Separate Debug and Release Roslyn cache entries so both rolling
configurations can seed immutable caches independently. Existing v1
entries remain a restore fallback.

## Measurements

The earlier Roslyn cache validation in #133748 measured
`libs.tests` at 4m53s with the cache versus 6m32s without it (25.25%
faster). Debug-specific hit rates cannot be measured until the first
rolling Debug build seeds its cache.

> [!NOTE]
> This pull request description was generated with GitHub Copilot.

Copilot-Session: 6b47078d-4fe1-4db0-93fd-264f9a651be7

Author: agocke

eng/pipelines/coreclr/templates/setup-roslyn-cache.yml

@@ -9,16 +9,18 @@ steps:
   # request itself or its target branch. This lets rolling builds seed the
   # cache without allowing pull requests to modify caches used by main.
   #
-  # The managed configuration is intentionally excluded. Roslyn's
-  # deterministic key includes the compilation options, and some pipeline
-  # legs override the matrix configuration in their build arguments.
+  # Keep Debug and Release entries separate so rolling builds can seed both
+  # configurations without competing for the same immutable cache key.
+  # Roslyn's own deterministic key accounts for legs that override the
+  # matrix configuration in their build arguments.
   - task: Cache@2
     displayName: Roslyn compiler cache
     continueOnError: true
     inputs:
-      key: roslyn | v1 | ${{ parameters.osGroup }}${{ parameters.osSubgroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }} | "$(Build.BuildId)"
+      key: roslyn | v2 | ${{ parameters.osGroup }}${{ parameters.osSubgroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }} | "$(buildConfigUpper)" | "$(Build.BuildId)"
       path: $(Pipeline.Workspace)/.roslyn-cache
       restoreKeys: |
+        roslyn | v2 | ${{ parameters.osGroup }}${{ parameters.osSubgroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }} | "$(buildConfigUpper)"
         roslyn | v1 | ${{ parameters.osGroup }}${{ parameters.osSubgroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }}
 
   - script: |

eng/pipelines/runtime.yml

@@ -351,6 +351,25 @@ extends:
                 eq(stageDependencies.EvaluatePaths.evaluate_paths.outputs['SetPathVars_libraries.containsChange'], true),
                 eq(variables['isRollingBuild'], true))
 
+      # Seed the Debug compiler cache used by the CoreCLR_AllSubsets PR builds.
+      - ${{ if eq(variables.isRollingBuild, true) }}:
+        - template: /eng/pipelines/common/platform-matrix.yml
+          parameters:
+            jobTemplate: /eng/pipelines/common/global-build-job.yml
+            buildConfig: debug
+            platforms:
+            - linux_x64
+            - linux_musl_x64
+            - osx_arm64
+            jobParameters:
+              nameSuffix: CoreCLR_AllSubsets
+              buildArgs: -s clr+libs+libs.tests+host+packs -rc Release -c $(_BuildConfig) /p:ArchiveTests=true
+              timeoutInMinutes: 180
+              preBuildSteps:
+                - template: /eng/pipelines/coreclr/templates/setup-sccache.yml
+              postBuildSteps:
+                - template: /eng/pipelines/coreclr/templates/sccache-stats.yml
+
       - template: /eng/pipelines/common/platform-matrix.yml
         parameters:
           jobTemplate: /eng/pipelines/common/global-build-job.yml