This was a bit of a challenge to figure out. The spawning section of the squid wiki:
Squids spawn in water between layers 46 and sea level, in groups of up to 4 in the world generation and can be found in any biome.
This makes it sound like squid spawn in any biome. After failing to get your farm to spawn any squid with multiple changes, I started reading the spawn wiki. Part of the section on java edition pack spawning:
Desert and snowy biomes (except cold taiga) do not spawn animals other than rabbits.
Squid do not spawn in the desert other then during world generation.
Your problem is the biome it is built in. You will have to rebuild in another biome.
The same section of the wiki also contains:
Beach, stone beach, river, ocean, and mesa cannot spawn animals; only hostile mobs and squid.
You have a river biome right next to your current build:
I modified level.dat so spawning can occur, drained the river, created a structure of your current farm, and loaded that structure in the river biome:
As you see, this was successful. Lots of squid. The farm is not very effiecient as it requires the squid to swim down to the bottom to suffocate. Some farms use flowing water so the squid are pushed out of the water.
The bad news is that you have to tear down and rebuild to move it. The good news, you don't have to move it very far.
If you want to see it in operation first, here is a download link to the modified world files. It is named MySqwidFarmMod.zip and it is 2.4MB.
tl;dr Squid don't spawn in desert biomes other than during world generation.