refactor(ci): update nightly toolchain version for udeps

[boquan-fang]

Release Summary:

Resolved issues:

Description of changes:

Our daily CI run detects udeps failure in https://github.com/aws/s2n-quic/actions/runs/17919636533/job/50950891477.

error: failed to compile `cargo-udeps v0.1.59`, intermediate artifacts can be found at `/tmp/cargo-installHaX4sJ`.
To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.

Caused by:
  rustc 1.86.0-nightly is not supported by the following packages:
    cargo@0.91.0 requires rustc 1.88
    cargo@0.91.0 requires rustc 1.88
    cargo@0.91.0 requires rustc 1.88
    cargo-credential-libsecret@0.5.1 requires rustc 1.88
    cargo-util@0.2.23 requires rustc 1.88
    cargo-util-schemas@0.10.0 requires rustc 1.88
    crates-io@0.40.13 requires rustc 1.88

This PR updates the nightly toolchain version to 1.88.0 to fix the issue.

Call-outs:

Testing:

Test the toolchain version on my local machine:

nightly-2025-04-01-x86_64-unknown-linux-gnu installed - rustc 1.88.0-nightly (0b45675cf 2025-03-31)

CI should pass with udeps: https://github.com/aws/s2n-quic/actions/runs/17921929531/job/50958821699?pr=2826

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[boquan-fang]

QUIC Attack run: https://us-west-2.console.aws.amazon.com/codesuite/codebuild/003495580562/projects/quic-attack/build/quic-attack%3Ac3278c4a-3315-4526-944b-795d7badbb37/?region=us-west-2.

The QUIC attack job is not reporting the status back to Github, but it runs successfully on AWS. @maddeleine can use a override and merge to unblock the CI if you think it's appropriate.