BUG: Route runtests.sh ruff invocations through PY_EXE

runtests.sh calls every other formatter (black, isort, pylint, pytype)
via "${PY_EXE}" -m so it targets the same interpreter the
is_pip_installed() guard checked. The ruff invocations still called a
bare `ruff` off PATH, so is_pip_installed("ruff") could pass against
$PY_EXE while the invocation ran a different (or absent) ruff from
PATH. In a clean venv built per CONTRIBUTING.md with no editable
PATH entry, ./runtests.sh --codeformat fails with
"ruff: command not found" even though ruff is installed and
importable from $PY_EXE.

Reproduced: with $PY_EXE pointed at a venv holding ruff and that
venv absent from PATH, the old `ruff --version` failed with
"command not found"; "${PY_EXE}" -m ruff --version succeeded.

Signed-off-by: Hans Johnson <hans-johnson@uiowa.edu>

Author: hjmjohnson

runtests.sh

@@ -595,13 +595,13 @@ then
     then
         install_deps
     fi
-    ruff --version
+    "${PY_EXE}" -m ruff --version
 
     if [ $doRuffFix = true ]
     then
-        ruff check --fix --unsafe-fixes --exclude versioneer.py --exclude "monai/_version.py" "$homedir"
+        "${PY_EXE}" -m ruff check --fix --unsafe-fixes --exclude versioneer.py --exclude "monai/_version.py" "$homedir"
     else
-        ruff check --exclude versioneer.py --exclude "monai/_version.py" "$homedir"
+        "${PY_EXE}" -m ruff check --exclude versioneer.py --exclude "monai/_version.py" "$homedir"
     fi
 
     ruff_status=$?