Jenkinsfile: add 'git fetch' to failsafe target

It was failing occationally with:
14:12:43  CMake Error at CMakeLists.txt:129 (list):
14:12:43    list index: 1 out of range (-1, 0)
14:12:43
14:12:43
14:12:43  CMake Error at CMakeLists.txt:131 (list):
14:12:43    list index: 2 out of range (-1, 0)
14:12:43
14:12:43
14:12:43  CMake Error at CMakeLists.txt:132 (string):
14:12:43    string sub-command REPLACE requires at least four arguments.
14:12:43
14:12:43
14:12:43  -- PX4 version: f513c40 (f513c40..NOTFOUND)

Author: bkueng

Jenkinsfile

@@ -105,6 +105,7 @@ pipeline {
             ./emsdk activate latest;
             cd ..;
             . ./_emscripten_sdk/emsdk_env.sh;
+            git fetch --all --tags;
             make failsafe_web;
             cd build/px4_sitl_default_failsafe_web;
             mkdir -p failsafe_sim;