nixos: Add sqlite to LD_LIBRARY_PATH in shell.nix

Otherwise a few of our tests fail with an error like (trimmed
and wrapped):

```
00:06 +256 -1: test/model/database_test.dart: non-migration tests create account [E]
  Invalid argument(s): Failed to load dynamic library 'libsqlite3.so':
    libsqlite3.so: cannot open shared object file: No such file or directory
```

(This didn't always happen, but the regression may have been a
while ago; I do only part of my development on NixOS, and I've been
tolerating this there for a bit.)

Author: gnprice

shell.nix

@@ -41,5 +41,6 @@ mkShell {
 
   LD_LIBRARY_PATH = lib.makeLibraryPath [
     fontconfig.lib
+    sqlite.out
   ];
 }