sw:vec_iree: fix mnist input image path

Following the change in https://github.com/google/iree/pull/8958,
update the mnist input image path to a fixed URL.

Change-Id: Ide74c0dfaadc6816f9ea7a3f400e4080bc5bc5ea

samples/float_model/CMakeLists.txt

diff --git a/samples/float_model/CMakeLists.txt b/samples/float_model/CMakeLists.txt
index d89bcc9..0903a84 100644
--- a/samples/float_model/CMakeLists.txt
+++ b/samples/float_model/CMakeLists.txt
@@ -53,8 +53,8 @@
   SHAPE
     "1, 28, 28, 1"
   SRC
-    "https://raw.githubusercontent.com/google/iree/main/ \
-    iree/samples/vision/mnist_test.png"
+    "https://github.com/google/iree/raw/ \
+    1e8d1fa96057c47d1fda918f3a2f71f14b073937/samples/iree_vision_inference/mnist_test.png"
   RANGE
     "0, 1"
 )