Fix person detect converter script (#2614)

BUG=https://github.com/tensorflow/tflite-micro/issues/2609

tensorflow/lite/micro/tools/make/ext_libs/person_detection_int8_vela_convert.sh

diff --git a/tensorflow/lite/micro/tools/make/ext_libs/person_detection_int8_vela_convert.sh b/tensorflow/lite/micro/tools/make/ext_libs/person_detection_int8_vela_convert.sh
index 0d9df87..b3fe24e 100755
--- a/tensorflow/lite/micro/tools/make/ext_libs/person_detection_int8_vela_convert.sh
+++ b/tensorflow/lite/micro/tools/make/ext_libs/person_detection_int8_vela_convert.sh
@@ -60,8 +60,7 @@
     TEMPFILE=$(mktemp -d)/
     python3 -m venv $TEMPFILE
     source $TEMPFILE/bin/activate
-    python3 -m pip install --upgrade pip >&2
-    pip install --upgrade cython >&2
+    pip install numpy==1.26.4 >&2
     pip install --prefer-binary ethos-u-vela >&2
   fi