[fix] morty & filtron: don't use golang installed by package manager

When golang is installed via a package manager the local version, which filtron
downloads, is ignored.

BTW: With the new go1.17.2 (cfea51f4), 'go get' is depricated::

  go get: installing executables with 'go get' in module mode is deprecated.
    Use 'go install pkg@version' instead.
    For more information, see https://golang.org/doc/go-get-install-deprecation
    or run 'go help get' or 'go help install'.

Reported-by: @tiekoetter https://github.com/searxng/searxng/pull/455#issuecomment-954918411


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