Add support for custom container for Linux Arm64

Update platform_matrix.yml to allow custom containers for Arm64 Linux runs.
This is required for our performance runs as we need a newer version of the
python runtime. This change follows the same pattern as https://github.com/dotnet/runtime/pull/59202/files

eng/pipelines/common/platform-matrix.yml

@@ -65,7 +65,10 @@ jobs:
       targetRid: linux-arm64
       platform: Linux_arm64
       container:
-        image: ubuntu-16.04-cross-arm64-20210719121212-8a8d3be
+        ${{ if eq(parameters.container, '') }}:
+          image: ubuntu-16.04-cross-arm64-20210719121212-8a8d3be
+        ${{ if ne(parameters.container, '') }}:
+          image: ${{ parameters.container }}
         registry: mcr
       jobParameters:
         runtimeFlavor: ${{ parameters.runtimeFlavor }}

eng/pipelines/coreclr/perf_slow.yml

@@ -98,6 +98,7 @@ jobs:
     parameters:
       jobTemplate: /eng/pipelines/common/build-coreclr-and-libraries-job.yml
       buildConfig: release
+      container: ubuntu-18.04-cross-arm64-20211022152824-b2c2436
       platforms:
       - Linux_arm64
       - windows_arm64
@@ -154,6 +155,9 @@ jobs:
       runtimeFlavor: aot
       platforms:
       - Linux_arm64
+      container:
+        image: ubuntu-18.04-cross-20211022152824-f853169
+        registry: mcr
       jobParameters:
         testGroup: perf
         liveLibrariesBuildConfig: Release
@@ -172,6 +176,7 @@ jobs:
       runtimeFlavor: coreclr
       platforms:
       - Linux_arm64
+      container: ubuntu-18.04-cross-arm64-20211022152824-b2c2436
       jobParameters:
         testGroup: perf
         liveLibrariesBuildConfig: Release