chore: add beta-4 toolchain #273

Job	Run time
nix-fmt-check	23s
nix-build-book	26s
nix-build (fuel, ubuntu-latest)	25s
nix-build (fuel, macos-latest)	1m 59s
nix-build (fuel, macos-arm)	12s
nix-build (fuel-beta-1, ubuntu-latest)	25s
nix-build (fuel-beta-1, macos-latest)	1m 42s
nix-build (fuel-beta-1, macos-arm)	11s
nix-build (fuel-beta-2, ubuntu-latest)	23s
nix-build (fuel-beta-2, macos-latest)	1m 39s
nix-build (fuel-beta-2, macos-arm)	11s
nix-build (fuel-beta-3, ubuntu-latest)	25s
nix-build (fuel-beta-3, macos-latest)	1m 33s
nix-build (fuel-beta-3, macos-arm)	10s
nix-build (fuel-beta-4-rc, ubuntu-latest)	21s
nix-build (fuel-beta-4-rc, macos-latest)	1m 39s
nix-build (fuel-beta-4-rc, macos-arm)	10s
nix-build (fuel-beta-4-rc-2, ubuntu-latest)	22s
nix-build (fuel-beta-4-rc-2, macos-latest)	1m 37s
nix-build (fuel-beta-4-rc-2, macos-arm)	10s
nix-build (fuel-beta-4, ubuntu-latest)	27s
nix-build (fuel-beta-4, macos-latest)	1m 42s
nix-build (fuel-beta-4, macos-arm)	11s
nix-build (fuel-nightly, ubuntu-latest)	28s
nix-build (fuel-nightly, macos-latest)	1m 39s
nix-build (fuel-nightly, macos-arm)	10s
nix-build (sway-vim, ubuntu-latest)	26s
nix-build (sway-vim, macos-latest)	1m 41s
nix-build (sway-vim, macos-arm)	10s
nix-develop	1s
	21m 18s

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