install.sh: use debian dist libffi

[3nprob]

We can use libffi-dev and libffi8 debian packages instead of downloading and building from source. Significantly reduces build time.

• bookworm/oldstable: 3.4.4 (https://packages.debian.org/bookworm/libffi-dev)
• trixie/stable: 3.4.8 (https://packages.debian.org/trixie/libffi-dev)

fall back to installing libffi from source like before on non-debian targets

[3nprob]

As for the others:

• libsecp256k1 has matching version for trixie but out of date for bookworm (which I guess should still be supported for now)
• libsodium stuck on 1.0.18 in debian (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1098271)

[AdamISZ]

tACK 8675a29

(tested on debian 6.1, full install plus running wallet-tool to check that nothing is broken).

to be clear, on libsodium our original thinking was that building from source represented less trust in external sources; but of course it's extremely debatable.

I don't have a strong opinion on it.

[kristapsk]

ACK 8675a29

[3nprob]

tACK 8675a29

(tested on debian 6.1, full install plus running wallet-tool to check that nothing is broken).

Thank you for verifying!

to be clear, on libsodium our original thinking was that building from source represented less trust in external sources; but of course it's extremely debatable.

Makes sense and I agree it's not obvious.

My 5c if it were my call, I'd hold off giving the same treatment for the other two until at least:

• libsodium: debian libsodium has any release (downgrading seems an unnecessary step back even if the dist version 1.0.18 probably works just as fine for most as it's what's in the last JM release)
• libsecp256k1: debian bookworm unsupported by joinmarket (the dist itself is set to be EoL 2026-06 which feels reasonable to match)