licenses: add Beerware

lib/licenses.nix

···
     fullName = "Boost Software License 1.0";
   };
 
+  beerware = spdx {
+    spdxId = "Beerware";
+    fullName = ''Beerware License'';
+  };
+
   bsd2 = spdx {
     spdxId = "BSD-2-Clause";
     fullName = ''BSD 2-clause "Simplified" License'';