[fix] make nvm.env exit with error when VERBOSE is unset

    $ make nvm.install
    INFO:  install (update) NVM at /800GBPCIex4/share/SearXNG/.nvm
    INFO:  already cloned at: /800GBPCIex4/share/SearXNG/.nvm
      || Fetching origin
    INFO:  checkout v0.39.1
      || HEAD is now at 9600617 v0.39.1
    make: *** [Makefile:96: nvm.install] Error 1

Without this fix we need to set VERBOSE environment to avoid the 'Error 1':

    $ VERBOSE=0 make nvm.install

BTW: fix an issue if there are any leftovers in ${NVM_DIR} from previos
installations

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>