NXP Backend: Use --index-url for eiq.nxp.com/repository (#13759)

### Summary
Avoid attack on PyPI index. If `--extra-index-url` is used there is an
attack vector when adversary publish a malicious package with same name
on pypi.org. And pip might pick the malicious package instead of one on
eiq.nxp.com/repository, as "there is no priority in the locations that
are searched" [https://pip.pypa.io/en/stable/cli/pip_install/]
Using --index-url eliminates this attack. 

### Test plan
N/A

Author: robert-kalmar

.github/workflows/pull.yml

@@ -855,7 +855,8 @@ jobs:
         .ci/scripts/setup-linux.sh --build-tool "cmake"
 
         # Install test requirements
-        pip install -r backends/nxp/requirements-tests.txt
+        pip install -r backends/nxp/requirements-tests-pypi.txt
+        pip install -r backends/nxp/requirements-tests-eiq.txt
 
         # Run pytest
         PYTHON_EXECUTABLE=python bash backends/nxp/run_unittests.sh

backends/nxp/requirements-tests-eiq.txt

@@ -0,0 +1,2 @@
+--index-url https://eiq.nxp.com/repository
+neutron_converter_SDK_25_03

backends/nxp/requirements-tests-pypi.txt

@@ -0,0 +1,4 @@
+tensorflow==2.18.0
+pytest-mock
+tflite
+GvGen

backends/nxp/requirements-tests.txt

@@ -7,4 +7,4 @@
 set -u
 
 # Install neutron-converter
-pip install --extra-index-url https://eiq.nxp.com/repository neutron_converter_SDK_25_03
+pip install --index-url https://eiq.nxp.com/repository neutron_converter_SDK_25_03