clean.sh & build-package.sh: avoid errors in case chmod's target dir is not exist

clean.sh

@@ -6,5 +6,5 @@ set -e -u
 test -f $HOME/.termuxrc && . $HOME/.termuxrc
 : ${TERMUX_TOPDIR:="$HOME/.termux-build"}
 
-chmod +w -R $TERMUX_TOPDIR
+[ -d "$TERMUX_TOPDIR" ] && chmod +w -R $TERMUX_TOPDIR
 rm -Rf /data/* $TERMUX_TOPDIR

scripts/build/termux_step_start_build.sh

@@ -94,7 +94,8 @@ termux_step_start_build() {
 
 	# Following directories may contain objects with RO-only permissions which
 	# makes them undeletable. We need fix that.
-	chmod +w -R "$TERMUX_PKG_BUILDDIR" "$TERMUX_PKG_SRCDIR"
+	[ -d "$TERMUX_PKG_BUILDDIR" ] && chmod +w "$TERMUX_PKG_BUILDDIR"
+	[ -d "$TERMUX_PKG_SRCDIR" ] && chmod +w "$TERMUX_PKG_SRCDIR"
 
 	# Cleanup old state:
 	rm -Rf "$TERMUX_PKG_BUILDDIR" \