set env vars for arm64 musl build

Author: Steve Lee (POWERSHELL HE/HIM) (from Dev Box)

.pipelines/DSC-Official.yml

@@ -316,6 +316,9 @@ extends:
           env:
             ob_restore_phase: true
         - pwsh: |
+            $env:CC_aarch64_unknown_linux_musl='clang'
+            $env:AR_aarch64_unknown_linux_musl='llvm-ar'
+            $env:CARGO_TARGET_AARCH64_UNKNOWN_LINUX_MUSL_RUSTFLAGS="-Clink-self-contained=yes -Clinker=rust-lld"
             apt -y install musl-tools
             ./build.ps1 -Release -Architecture aarch64-unknown-linux-musl
             ./build.ps1 -PackageType tgz -Architecture aarch64-unknown-linux-musl -Release