Update to reflex-platform staging branch with nixpkgs-21.05

[ali-abrar]

Depends on https://github.com/reflex-frp/reflex-platform/pull/768. This can be taken out of draft state when that gets merged, which depends on the resolution of these issues.

I have:

• [x] Based work on latest develop branch
• [ ] Followed the contribution guide
• [ ] Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• [ ] Run the test suite: $(nix-build -A selftest --no-out-link)
• [ ] Updated the changelog
• [ ] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[ali-abrar]

As of https://github.com/obsidiansystems/obelisk/pull/972/commits/fe415ecb22845611f48b39eef2e4984a0375ac59 ob run works on this branch

[luigy]

Closing this as it has been merged as part of https://github.com/obsidiansystems/obelisk/pull/1005