[Nix] Provide default for squadcal_url.json

Summary:
Add squalcal_url.json default with other facts.

squalcal_url.json is being used a fallback router for "/" in
this regard.

May be redundant with D5846, but aligns with intent.

https://linear.app/comm/issue/ENG-2260

Depends on D5846

Test Plan:

rm keyserver/facts/*.json

nix develop

# Assert files were written, and content matches scripts/templates
ls keyserver/facts
cat keyserver/facts/*.json

Reviewers: atul, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5847