update rust nightly version to 2026-04-17

This matches rust-bitcoin.

Author: apoelstra

Cargo.toml

@@ -11,7 +11,7 @@ edition = "2018"
 rust-version = "1.63.0"
 
 [workspace.metadata.rbmt.toolchains]
-nightly = "nightly-2025-09-27"
+nightly = "nightly-2026-04-17"
 stable = "1.95.0"
 
 [features]