Don't call cmd.exe if not needed

[ryncsn]

Hi, I found starting new subshell in WSL2 with this Fedora remix is really slow, and found the reason is caused by this 00-remix.sh, I tried to fix it, as long as WIN_HOME is not a variable that changes from time to time, this fix should be valid, please have a look.

cmd.exe is called in 00-remix.sh to get and set the WIN_HOME variable.

When spawning multiple shell instances inside a already running interactive
shell, this cmd.exe call slows down the startup speed by a lot, skip this call
if WIN_HOME is already set.

[crramirez]

Thank you for your contribution. It looks like great

[leonbloy]

This almost fixes my issue #129
but with two caveats:

1. I think it's not foolproof to rely on the definition of $WIN_HOME
   It will not be available if setting (as was my case)

[interop]
appendWindowsPath=false

I would be better to define some ad-hoc env variable like WSL_00_REMIX_DONE=1 or something like that

2. We want to avoid the unnecesary heavy tasks in child shells. I think it would be better to except, not only
   the block that starts with the comment # Check if we have Windows Path
   but also the lines

setup_display
# if dbus-launch is installed then load it
if (command -v dbus-launch >/dev/null 2>&1); then
  eval "$(timeout 2s dbus-launch --auto-syntax)"
fi