fetchTarball needs a sha256

[palfrey]

Tried adding a fetchTarball variant to my config (mostly to avoid a really long git clone), and got error: in pure evaluation mode, 'fetchTarball' requires a 'sha256' argument

[lazamar]

Ok, so it looks like that's really needed in pure mode.

If you are just installing stuff locally, apparently you can use the --impure flag to get rid of the error.

But it might be nice to provide a sha256 of the tarball so that the install command can work in pure mode.

Right now we tell nix to fetch this tarball and use it at indexing time.

nix-package-versions/src/Nix.hs

Lines 193 to 197 in 3b7d0c6

	command destination =
	"nix-env -qaP --json -f "
	<> GitHub.archiveUrl nixpkgsRepo commit
	<> " --arg config '" <> config <> "'"
	<> " >" <> destination

We could instead download the tarball during indexing and tell Nix to use the downloaded result, this way we could also get the sha256 of the file along the way.

Tell me if using --impure would work for your use case.