protobuf should use PYEXE not pip (#5231)

### Description

We should be using `PY_EXE` in runtests as opposed to raw `pip`.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).

Signed-off-by: Richard Brown <33289025+rijobro@users.noreply.github.com>

Author: rijobro

runtests.sh

@@ -14,13 +14,6 @@
 # script for running all tests
 set -e
 
-# FIXME: https://github.com/Project-MONAI/MONAI/issues/4354
-protobuf_major_version=$(pip list | grep '^protobuf ' | tr -s ' ' | cut -d' ' -f2 | cut -d'.' -f1)
-if [ "$protobuf_major_version" -ge "4" ]
-then
-    export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python
-fi
-
 # output formatting
 separator=""
 blue=""
@@ -118,6 +111,13 @@ function print_usage {
     exit 1
 }
 
+# FIXME: https://github.com/Project-MONAI/MONAI/issues/4354
+protobuf_major_version=$(${PY_EXE} -m pip list | grep '^protobuf ' | tr -s ' ' | cut -d' ' -f2 | cut -d'.' -f1)
+if [ "$protobuf_major_version" -ge "4" ]
+then
+    export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python
+fi
+
 function check_import {
     echo "Python: ${PY_EXE}"
     ${cmdPrefix}${PY_EXE} -W error -W ignore::DeprecationWarning -c "import monai"