stage-1: Clear environment before calling stage-2

In particular, stage-1's LD_LIBRARY_PATH should not infect stage-2.
Reported by viric.

modules/system/boot/stage-1-init.sh

@@ -365,6 +365,6 @@ mount --move /sys $targetRoot/sys
 mount --move /dev $targetRoot/dev
 mount --move /run $targetRoot/run
 
-exec switch_root "$targetRoot" "$stage2Init"
+exec env -i $(type -P switch_root) "$targetRoot" "$stage2Init"
 
 fail # should never be reached

modules/system/boot/systemd.nix

@@ -224,6 +224,7 @@ let
 
           [Service]
           Environment=PATH=${def.path}
+          Environment=LD_LIBRARY_PATH=
           ${let env = cfg.globalEnvironment // def.environment;
             in concatMapStrings (n: "Environment=${n}=${getAttr n env}\n") (attrNames env)}
           ${optionalString (!def.restartIfChanged) "X-RestartIfChanged=false"}