Switch to command -v over which

This fixes an issue on macOS where `which` failed to return any results
because the `PATH` wasn't propagated. `command` is more standard for
this use case as well https://github.com/koalaman/shellcheck/wiki/SC2230

Fixes bazelbuild#8414

tools/python/pywrapper_template.txt

@@ -16,10 +16,10 @@ die() {
 }
 
 # Try the "python%VERSION%" command name first, then fall back on "python".
-PYTHON_BIN=`which python%VERSION% || echo ""`
+PYTHON_BIN=`command -v python%VERSION% || echo ""`
 USED_FALLBACK=0
 if [ -z "${PYTHON_BIN:-}" ]; then
-    PYTHON_BIN=`which python || echo ""`
+    PYTHON_BIN=`command -v python || echo ""`
     USED_FALLBACK=1
 fi
 if [ -z "${PYTHON_BIN:-}" ]; then