Test nix GC

[alexfmpe]

Write a test to check whether nix-collect-garbage is deleting things we don't want it to

[ali-abrar]

What is supposed to be kept around? Is try-reflex meant to add gc roots?

[alexfmpe]

Well, the idea is checking that doing something like

./try-reflex
nix-collect-garbage
./try-reflex

would not build anything on the second try-reflex

Is try-reflex meant to add gc roots?

Yes, as per https://github.com/reflex-frp/reflex-platform/commit/065775769e1e712cc5be9718a7f8fd588e941971

At first, the nix 1.12 beta started ignoring --add-root and rejecting --indirect arguments in nix-build. If they had been both ignored, we wouldn't have noticed our gc roots were no longer effective, hence this test.

Currently there's still a fair bit of re-downloading which is I believe is due to https://github.com/NixOS/nixpkgs/issues/36190