fix(ci): freezing cpp-jwt to v1.4

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

build/scripts/build_helper.nrf

@@ -355,7 +355,7 @@ install_cpp_jwt_from_git() {
     fi
 
     git clone $GIT_URL
-    cd cpp-jwt && git checkout master
+    cd cpp-jwt && git checkout -f v1.4
     ret=$?;[[ $ret -ne 0 ]] && popd && return $ret
     mkdir _build && cd _build
     $CMAKE ..