shell/caucase.sh: Give precedence to CAUCASE_PYTHON.

If environment points us at a specific python interpreter, use it to run
caucased, even if caucased is present reachable from PATH.

shell/caucase.sh

@@ -1199,11 +1199,11 @@ EOF
       exit 1
     }
     echo 'Automated testing...'
-    if command -v caucased > /dev/null; then
-      caucased_type="path"
-    elif [ "x${CAUCASE_PYTHON:-}" != "x" ] && [ -x "$CAUCASE_PYTHON" ]; then
+    if [ "x${CAUCASE_PYTHON:-}" != "x" ] && [ -x "$CAUCASE_PYTHON" ]; then
       # Used when ran from python caucase.test
       caucased_type="environment"
+    elif command -v caucased > /dev/null; then
+      caucased_type="path"
     else
       echo 'caucased not found in PATH, cannot run tests' >&2
       return 1