#1929 fix(ci): free more disk space and add nix fallback for cache mismatches

Summary

• Enable tool-cache: true and explicit category flags in free-disk-space action for both E2E and Docker workflows to reclaim ~5GB additional disk space (fixes No space left on device during nix build on Ubuntu)
• Add --fallback to NIX_FLAGS in Makefile and fallback = true to nix.conf in Dockerfile and E2E workflow, so nix builds from source when binary cache returns hash mismatches (fixes arm64 Docker build failure: hash mismatch importing path .../cyrus-sasl-2.1.28-dev)

Test plan

• E2E (Ubuntu) job completes without disk space exhaustion
• Docker (linux/arm64) job completes without nix cache hash mismatch errors
• E2E (MacOS) and E2E (NixOS) jobs remain green
• Docker (linux/amd64) job remains green

🤖 Generated with Claude Code

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Summary by cubic

Free up more disk space on Linux runners and add Nix fallback to fix CI build failures. This prevents “No space left on device” and handles cache hash mismatches, stabilizing E2E and Docker builds (including linux/arm64).

• Bug Fixes
  • E2E and Docker workflows: enable tool-cache: true and explicit categories in jlumbroso/free-disk-space@v1.3.1 to reclaim ~5 GB.
  • Nix: set fallback = true in nix.conf (Dockerfile and E2E) and add --fallback to NIX_FLAGS in the Makefile so builds continue from source on cache mismatches.