ci: pin nightly toolchain to 2026-04-16 for ethnum 1.5.2 compat

[LuciferYang]

Summary

The linux-build CI job installs unpinned nightly, which broke after 2026-04-17 because ethnum 1.5.2 uses unsafe { mem::transmute(()) } to create TryFromIntError. Newer nightly builds reject this with error[E0512]: cannot transmute between types of different sizes.

Dependency chain: lance-arrow → jsonb 0.5.6 → ethnum 1.5.2

This PR pins nightly-2026-04-16 (last known-good date) in both the toolchain install step and the cargo +nightly invocation.

Root Cause

ethnum-1.5.2/src/error.rs:16:

pub const fn tfie() -> TryFromIntError {
    unsafe { mem::transmute(()) }  // () is 0 bits, TryFromIntError is 8 bits
}

Rust nightly e9e32aca5 (2026-04-17) tightened transmute checks, making this a hard error.

Follow-up

• Upstream fix needed in nlordell/ethnum-rs to replace the transmute hack
• Once ethnum publishes a fix and jsonb picks it up, the pin can be removed

Test plan

• linux-build job passes with pinned nightly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[LuciferYang]

For the medium-term fix, we've submitted an upstream PR to ethnum-rs: nlordell/ethnum-rs#58

The root cause is that ethnum v1.5.2 uses unsafe { mem::transmute(()) } to create a TryFromIntError. Starting with Rust nightly-2026-04-18 on Linux x86_64, TryFromIntError grew from 0 to 1 byte, causing a compile-time E0512 size mismatch error. The fix replaces both transmute hacks (tfie() and pie()) with safe stdlib alternatives.

Once the upstream fix is released, we can unpin the nightly version in this PR.

[jackye1995]

Thanks for the fix!

[LuciferYang]

Thank you @jackye1995 ~