Update SourceBuild image to be our Centos 8 Stream image. (#83106)

eng/pipelines/common/global-build-job.yml

@@ -102,12 +102,6 @@ jobs:
         ${{ if notin(parameters.osGroup, 'ios', 'tvos', 'maccatalyst')}}:
           value: ''
 
-      - name: _sclEnableCommand
-        ${{ if eq(parameters.isSourceBuild, true) }}:
-          value: scl enable llvm-toolset-7.0 --
-        ${{ if ne(parameters.isSourceBuild, true) }}:
-          value: ''
-
       - name: _monoAotBuildshCommand
         value: ''
 
@@ -166,7 +160,7 @@ jobs:
       - template: /eng/common/templates/steps/source-build.yml
         parameters:
           platform:
-            buildScript: $(_sclEnableCommand) $(Build.SourcesDirectory)$(dir)build$(scriptExt)
+            buildScript: $(Build.SourcesDirectory)$(dir)build$(scriptExt)
             nonPortable: ${{ parameters.isNonPortableSourceBuild }}
             targetRID: ${{ parameters.targetRid }}
             runtimeOS: linux
@@ -194,7 +188,7 @@ jobs:
         - task: CodeQL3000Init@0
           displayName: Initialize CodeQL (manually-injected)
 
-      - script: $(_sclEnableCommand) $(Build.SourcesDirectory)$(dir)build$(scriptExt) -ci -arch ${{ parameters.archType }} $(_osParameter) ${{ parameters.buildArgs }} $(_officialBuildParameter) $(_crossBuildPropertyArg) $(_cxx11Parameter) $(_buildDarwinFrameworksParameter) $(_overrideTestScriptWindowsCmdParameter)
+      - script: $(Build.SourcesDirectory)$(dir)build$(scriptExt) -ci -arch ${{ parameters.archType }} $(_osParameter) ${{ parameters.buildArgs }} $(_officialBuildParameter) $(_crossBuildPropertyArg) $(_cxx11Parameter) $(_buildDarwinFrameworksParameter) $(_overrideTestScriptWindowsCmdParameter)
         displayName: Build product
         ${{ if eq(parameters.useContinueOnErrorDuringBuild, true) }}:
           continueOnError: ${{ parameters.shouldContinueOnError }}

eng/pipelines/common/templates/pipeline-with-resources.yml

@@ -44,8 +44,10 @@ resources:
       env:
         ROOTFS_DIR: /crossrootfs/x86
 
+    # CentOS 8 Stream is the closest image to RHEL8, which has the oldest toolsets we support building against
+    # for our source-build partners.
     - container: SourceBuild_linux_x64
-      image: mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7-source-build
+      image: mcr.microsoft.com/dotnet-buildtools/prereqs:centos-stream8
 
     - container: linux_s390x
       image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-s390x