fetch-configlet: explicitly pass -s to uname (#808)

I believe that a bare `uname` should do the same thing as `uname -s`
everywhere. But let's not rely on the reader knowing this, and instead
better communicate that we want the operating system name.

With GNU coreutils 9.3:

    $ uname --help
    Usage: uname [OPTION]...
    Print certain system information.  With no OPTION, same as -s.

      -a, --all                print all information, in the following order,
                                 except omit -p and -i if unknown:
      -s, --kernel-name        print the kernel name
    [...]

And on OpenBSD [1]:

    If no options are specified, uname prints the operating system name as if the -s option had been specified.

[1] https://man.openbsd.org/uname

scripts/fetch-configlet

@@ -48,7 +48,7 @@ main() {
   fi
 
   local os
-  case "$(uname)" in
+  case "$(uname -s)" in
     Darwin*)   os='macos'   ;;
     Linux*)    os='linux'   ;;
     Windows*)  os='windows' ;;