packaging-nix: work around direnv bug

Restore the TMP, TEMP, TMPDIR, TEMPDIR variables to their previous
values if `use flake` modified them in `.envrc`.

Signed-off-by: Philipp Jungkamp <Philipp.Jungkamp@opal-rt.com>

.envrc

@@ -1,7 +1,34 @@
+
+export_or_unset()
+{
+	local var=$1
+
+	if [ -z "${!var+x}" ]; then
+		return
+	fi
+
+	if [ -n "$2" ]; then
+		export $var="$2"
+	else
+		unset $var
+	fi
+
+}
+
 if direnv_version "2.30.0" \
 && has nix \
 && nix show-config experimental-features 2>/dev/null | grep -wqF flakes
 then
-    watch_file ./packaging/nix/*.nix
-    use flake ./packaging/nix
+	local oldtmp="$TMP"
+	local oldtemp="$TEMP"
+	local oldtmpdir="$TMPDIR"
+	local oldtempdir="$TEMPDIR"
+
+	watch_file ./packaging/nix/*.nix
+	use flake ./packaging/nix
+
+	export_or_unset TMP "$oldtmp"
+	export_or_unset TEMP "$oldtemp"
+	export_or_unset TMPDIR "$oldtmpdir"
+	export_or_unset TEMPDIR "$oldtempdir"
 fi