[mlir][SparseTensor][ArmSVE] Disable scalable vectorisation in a test

The MLIR SVE integration tests are now enabled in the
clang-aarch64-full-2stage buildbot under emulation (QEMU) and one of the
sparse integration tests is failing [1]:

  * Integration/Dialect/SparseTensor/CPU/concatenate_dim_1.mlir

That test is failing because we we don't have a LIT substitution to
replace:
  ```
  ; RUN: mlir-cpu-runner <command>
  ```
with
  ```
  ; RUN: <emulator> mlir-cpu-runner <command>
  ```
clang-aarch64-full-2stage does not support SVE natively and hence all
SVE integration tests require emulation. Other SVE tests use `lli` (for
which we do have the required substitution) and hence are not affected.

This patch simplifies concatenate_dim_1.mlir to always use fixed-width
vectorisation. We will re-enable scalable vectorisation once LIT
substitutions for `mlir-cpu-runner` are updated.

[1] https://lab.llvm.org/buildbot/#/builders/179/builds/6062