fix: disable SponsoredFPC on staging-public (#22150)

## Summary

Disables the SponsoredFPC contract in the staging-public environment to
prevent genesis archive root divergence across aztec-nr versions.

The SponsoredFPC contract address depends on its compiled bytecode,
which changes whenever aztec-nr is updated (oracle renames, log domain
separation, etc.). This causes the genesis archive root to differ
between versions, bricking nodes that were deployed with a different
aztec-nr version.

Setting `SPONSORED_FPC=false` aligns staging-public with testnet and
mainnet configuration, which don't use SponsoredFPC.

## Context

- Nodes running 4.2.0-aztecnr.2 on staging-public were bricked because
the genesis archive root changed
- Root cause: aztec-nr breaking changes recompile SponsoredFPC →
different address → different genesis root
- See analysis:
https://gist.github.com/AztecBot/21c76a7d939dcff5af52e6fc23d6db76

## Test plan

- [ ] Verify staging-public redeploys successfully without SponsoredFPC
- [ ] Confirm genesis archive root matches across v4 and v4-next
versions"

ClaudeBox log: https://claudebox.work/s/dd9d82043dbc175f?run=4

Author: AztecBot

spartan/environments/staging-public.env

@@ -24,7 +24,7 @@ TX_COLLECTION_FILE_STORE_URLS="https://aztec-labs-snapshots.com/${TX_FILE_STORE_
 R2_ACCESS_KEY_ID=REPLACE_WITH_GCP_SECRET
 R2_SECRET_ACCESS_KEY=REPLACE_WITH_GCP_SECRET
 TEST_ACCOUNTS=false
-SPONSORED_FPC=true
+SPONSORED_FPC=false
 
 # Match testnet/mainnet mana target (75M) instead of global default (100M).
 # AZTEC_MANA_TARGET only takes effect on L1 contract redeployment.