rust/bindgen: use temp file to generating bindings

Prefixing a file with sed doesn't appear to be portable. Instead, make
use of a temporary file.

Fixes generating the bindings on FreeBSD and Mac.

rust/Makefile.am

@@ -108,7 +108,7 @@ vendor:
 update-bindings:
 if HAVE_BINDGEN
 	$(BINDGEN) \
-		-o sys/src/sys.rs \
+		-o sys/src/sys.rs.tmp \
 		--rust-target 1.68 \
 		--no-layout-tests \
 		--disable-header-comment \
@@ -120,7 +120,9 @@ if HAVE_BINDGEN
 		$(abs_top_srcdir)/src/bindgen.h \
 		-- \
 		-DHAVE_CONFIG_H -I../src -I../rust/gen $(CPPFLAGS)
-	sed -i '1i\// This file is automatically generated. Do not edit.\n' sys/src/sys.rs
+	printf "// This file is automatically generated. Do not edit.\n\n" > sys/src/sys.rs
+	cat sys/src/sys.rs.tmp >> sys/src/sys.rs
+	rm -f sys/src/sys.rs.tmp
 else
 	@echo "error: bindgen not installed, can't update bindings"
 	exit 1