nginx: increase types_hash_max_size to 4096 (#83609)

After upgrading to NixOS 20.03, I've got the following warning:

    nginx: [warn] could not build optimal types_hash, you should increase either types_hash_max_size: 2048 or types_hash_bucket_size: 64; ignoring types_hash_bucket_size

The documentation states that "if nginx emits the message requesting
to increase either hash max size or hash bucket size then the first
parameter should first be increased" (aka types_hash_max_size).

In 19.03, the size of mime.types was around 100 entries. In 20.03, we
are around 900 entries. This is due to ff0148d8 which makes nginx
use mailcap mime.types.