remove non-reproducible nightly dependency

[oech3]

I was able to build this with RUSTC_BOOTSTRAP=1 on Arch Linux rust 1:1.95.0.
Also it seems if let guard is already stable rust-lang/rust#51114

[Alonely0]

I actually thought it ran on stable 1.91.0 for this reason, but Sylvestre had some issues and turned on nightly. I'll investigate.