Enable sccache for linux-musl-x64 coreclr builds (#128050)

> [!NOTE]
> This PR was generated with the assistance of GitHub Copilot.

## Summary

Extends sccache caching from linux-x64 to also cover linux-musl-x64
coreclr build legs. Based on the linux-x64 results (~9 min/leg gross
savings, 97.59% hit rate at steady state), this should provide similar
speedups for the musl legs.

## Changes

- **setup-sccache.yml**: Widen condition to include `osSubgroup ==
'_musl'`; add `osSubgroup` to cache key to keep musl/glibc caches
separate
- **sccache-stats.yml**: Widen condition to match

## Why this works

- The sccache NuGet package (v0.15.0) is a **statically-linked** binary
— verified with `file` showing `static-pie linked`. Works on both glibc
and musl.
- The sccache package is already downloaded by `runtime-prereqs.proj` on
all Linux CI builds (line 13-14, condition: `IsOsPlatform(Linux)`).
- Cache keys include `osSubgroup` so musl builds get
`sccache|linux_musl|x64|...` vs `sccache|linux|x64|...` for glibc — no
cross-contamination.
- For non-musl builds, `osSubgroup` is empty, so the key is
**unchanged** — existing caches remain valid.

## Also included

`SCCACHE_CACHE_SIZE=3.5G` limit (from #128048) to prevent unbounded
cache growth.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: agocke

eng/pipelines/coreclr/templates/sccache-stats.yml

@@ -12,7 +12,7 @@ parameters:
   osSubgroup: ''
 
 steps:
-  - ${{ if and(eq(parameters.osGroup, 'linux'), eq(parameters.osSubgroup, ''), eq(parameters.archType, 'x64')) }}:
+  - ${{ if and(eq(parameters.osGroup, 'linux'), or(eq(parameters.osSubgroup, ''), eq(parameters.osSubgroup, '_musl')), eq(parameters.archType, 'x64')) }}:
     - script: sccache --show-stats || true
       displayName: Sccache stats
       condition: always()

eng/pipelines/coreclr/templates/setup-sccache.yml

@@ -15,7 +15,7 @@ parameters:
   sccacheVersion: '0.15.0'
 
 steps:
-  - ${{ if and(eq(parameters.osGroup, 'linux'), eq(parameters.osSubgroup, ''), eq(parameters.archType, 'x64')) }}:
+  - ${{ if and(eq(parameters.osGroup, 'linux'), or(eq(parameters.osSubgroup, ''), eq(parameters.osSubgroup, '_musl')), eq(parameters.archType, 'x64')) }}:
     # Set up the Azure Pipeline Cache for sccache's local cache directory.
     # Use a rolling key so each build can update the cache; restoreKeys
     # falls back to the most recent saved entry.
@@ -30,10 +30,10 @@ steps:
       displayName: Sccache cache
       continueOnError: true
       inputs:
-        key: sccache | ${{ parameters.osGroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }} | "$(Build.BuildId)"
+        key: sccache | ${{ parameters.osGroup }}${{ parameters.osSubgroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }} | "$(Build.BuildId)"
         path: $(Pipeline.Workspace)/.sccache
         restoreKeys: |
-          sccache | ${{ parameters.osGroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }}
+          sccache | ${{ parameters.osGroup }}${{ parameters.osSubgroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }}
 
     # Configure sccache environment and add binary to PATH.
     # The sccache package is restored by runtime-prereqs.proj during the build.