ARM-CI : Use archived root-fs and run tests with --sequential option (dotnet/coreclr...

ARM-CI : Use archived root-fs and run tests with --sequential option (dotnet/coreclr#7946)

* ARM-CI : Use archived root-fs and run tests with --sequential option

To make sure the reason of test failure ramdomly( dotnet/coreclr#6298 ),
We checked segmentation faults occurred from mounted rootfs and the multi thread processing.

So I changed root-fs to archived root-fs and run tests with --sequential option.

* change to original clang version

Commit migrated from https://github.com/dotnet/coreclr/commit/cd33bb684837bf351875d64af27cc270675eae98