Use different folder to checkout tt-mlir

Cmake expexts that folder used for external project is empty.
Use different folder to checkout tt-mlir

0.695 CMake Error at CMakeLists.txt:19 (message):
0.695   Failed to clone tt-mlir: fatal: destination path
0.695   '/home/build/tt-xla/third_party/tt-mlir/src/tt-mlir' already exists and is
0.695   not an empty directory.

.github/get-docker-tag.sh

@@ -12,7 +12,7 @@ MLIR_DOCKER_TAG=$(
     # Read tt-mlir version from third_party/CMakeLists.txt
     # clone tt-mlir version to tmp/third_party/tt-mlir
     # Get the MLIR docker tag
-    TT_MLIR_PATH=third_party/tt-mlir/src/tt-mlir
+    TT_MLIR_PATH=tmp/third_party/tt-mlir
     TT_MLIR_VERSION=$(grep -oP 'set\(TT_MLIR_VERSION "\K[^"]+' third_party/CMakeLists.txt)
     if [ ! -d $TT_MLIR_PATH ]; then
         git clone https://github.com/tenstorrent/tt-mlir.git $TT_MLIR_PATH --quiet