Add curl to Gitian setup instructions

[btcdrak]

curl is required to fetch dependencies

requires backport to all branches

[ci skip]

[MarcoFalke]

ACK

[luke-jr]

The gitian descriptors should be installing this now, although I suppose this saves time.

[MarcoFalke]

As I understand this, they are only installed on the guest system: https://github.com/devrandom/gitian-builder/blob/bb4f92f6cbde6ee78e39ae35b0934da3b55e154d/bin/gbuild#L97

So this pull seems like a good place to revert #7614?

[luke-jr]

They're only needed on the guest system.

[MarcoFalke]

They're only needed on the guest system.

Then I fail to see why this pull "requires backport to all branches" considering that Debian is the host system.

Edit: I see, it's required for the optional step in https://github.com/bitcoin/bitcoin/blob/master/doc/release-process.md#optional-seed-the-gitian-sources-cache-and-offline-git-repositories

[btcdrak]

Without this patch, make -C depends/... command fails to download the dependencies required for gitian builds.

[luke-jr]

@btcdrak But make -C depends/... is not part of the setup instructions... and inside the gitian environment curl is installed by the packages list.

[btcdrak]

@luke-jr It is required for this https://github.com/bitcoin/bitcoin/blob/master/doc/release-process.md#optional-seed-the-gitian-sources-cache-and-offline-git-repositories

[MarcoFalke]

Backported as part of #7938. Removing label 'Needs backport'.