Create and set permissions of files atomically

Apparently it is not safe to first create a file with touch and then set
permissions with chmod. An unprivileged user could open the file in
between. See
https://github.com/NixOS/nixpkgs/pull/60138#discussion_r623971755.

secrets

@@ -91,16 +91,13 @@ echo "!*data" >&2
 
 read basedir
 if test ! -d "$basedir"; then
-    mkdir "$basedir"
-    chown root:root "$basedir"
-    chmod 755 "$basedir"
+    install -o root -g root -m 755 -d "$basedir"
 fi
 rm -rf "$basedir"/*
 
 while read name; do
     fullpath="$basedir/$name"
-    touch "$fullpath"
-    chmod 600 "$fullpath"
+    install -o root -g root -m 600 /dev/null "$fullpath"
     read size
     head --bytes="$size" - > "$fullpath"
     read owner