Use Rust 1.88 during CI

[nojaf]

I’ve had some builds that install version 1.90 unexpectedly. Please pin it to version 1.88 until we intentionally move to a newer version. Any upgrade should be a conscious decision.

Failed build: https://github.com/rescript-lang/rescript/actions/runs/17830424349/job/50693695893#step:11:138

Notice

rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0
LLVM version: 20.1.8

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