nixpkgs: Update to latest revision

[YusukeShimizu]

Update nixpkgs to revision 6915a163f351c32bd4557518d047725665e83d37

• Update cln to v24.08.02
• Update lnd to v0.18.3-beta
• Update bitcoin to v28.0

[!NOTE] Only changes related to integration test code.

[grubles]

ACK 590081d