Add /dev/stdin

Add /dev/stdin because it is necessary when using genrule in external/cronet

Bug: 267379939
Test: build
Change-Id: Ie3fd6c875ce02514745caf7cf7502fca668cdcdb
diff --git a/build/sandbox/nsjail.cfg b/build/sandbox/nsjail.cfg
index 509ecc6..5341577 100644
--- a/build/sandbox/nsjail.cfg
+++ b/build/sandbox/nsjail.cfg
@@ -154,3 +154,10 @@
   dst: "/dev/zero"
   is_bind: true
 }
+
+# /dev/stdin used during the creation files in external/cronet
+mount {
+  src: "/proc/self/fd/0"
+  dst: "/dev/stdin"
+  is_symlink: true
+}