Check for `file` command in Tor Browser start script before using it

In start-tor-browser we do

SYSARCHITECTURE=$(getconf LONG_BIT)
TORARCHITECTURE=$(expr "$(file TorBrowser/Tor/tor)" : '.*ELF \([[:digit:]]*\)')

if [ $SYSARCHITECTURE -ne $TORARCHITECTURE ]; then
   complain "Wrong architecture? 32-bit vs. 64-bit."
   exit 1
fi

to bail out early in case users have downloaded a bundle for the wrong architecture. Now, it turns out that there are Linux distros out there (NixOS seems to be one of those) that don't find file that way. A fix for that would be to check for the existence of file and if we can't find it to note that we assume the user knows what they are doing and proceed anyway.