nixos-rebuild: Add proper escaping to ssh calls

SSH merges its arguments by space-concatenation - it does not preserve
the array structure.

This is arguably a historic mistake, whose fix would be too breaking.
I suppose it will stay this way forever, until perhaps a better behavior
can be opted in to using a flag, and I don't think this flag exists yet.

To make multi-argument commands work reliably over ssh, we need to escape
them, which is what the ${@@Q} incantation achieves.

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -220,9 +220,9 @@ buildHostCmd() {
     if [ -z "$buildHost" ]; then
         runCmd "$@"
     elif [ -n "$remoteNix" ]; then
-        runCmd ssh $SSHOPTS "$buildHost" "${c[@]}" env PATH="$remoteNix":'$PATH' "$@"
+        runCmd ssh $SSHOPTS "$buildHost" "${c[@]}" env PATH="$remoteNix":'$PATH' "${@@Q}"
     else
-        runCmd ssh $SSHOPTS "$buildHost" "${c[@]}" "$@"
+        runCmd ssh $SSHOPTS "$buildHost" "${c[@]}" "${@@Q}"
     fi
 }
 
@@ -237,7 +237,7 @@ targetHostCmd() {
     if [ -z "$targetHost" ]; then
         runCmd "${c[@]}" "$@"
     else
-        runCmd ssh $SSHOPTS "$targetHost" "${c[@]}" "$@"
+        runCmd ssh $SSHOPTS "$targetHost" "${c[@]}" "${@@Q}"
     fi
 }