wg-quick: freebsd: avoid writing private keys to /tmp

FreeBSD's bash doesn't handle <(...) safely, creating a temporary file
instead of using /proc/self/fd/N like on Linux. Work around this by
using a simple pipeline with /dev/stdin.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/wg-quick/freebsd.bash

@@ -349,7 +349,7 @@ add_route() {
 }
 
 set_config() {
-	cmd wg setconf "$INTERFACE" <(echo "$WG_CONFIG")
+	echo "$WG_CONFIG" | cmd wg setconf "$INTERFACE" /dev/stdin
 }
 
 save_config() {