fix(prqlc/deps): loosen chrono constraint from 0.4.44 to 0.4

[lukapeschke]

depending on the latest chrono version causes conflicts with other libs relying on older versions, such as polars: https://github.com/pola-rs/polars/blob/693c52035689e150c2c70fc6d48c952c564ec940/Cargo.toml#L44

[max-sixty]

Heads-up for context: this loosening exposed a transitive-dep ratchet that wasn't visible from prqlc/prqlc/Cargo.toml alone — arrow-arith (transitive via connector_arrow → arrow → arrow-arith) requires chrono ^0.4.40, and duckdb requires chrono ^0.4.22. So chrono = "0.4" is below what prqlc actually transitively requires, and cargo update -Z direct-minimal-versions (the test-deps-min-versions job) refuses to resolve.

It didn't show up in this PR's CI because the nightly path filter in tests.yaml only matched the workspace-root Cargo.toml, not member crates. So test-deps-min-versions was skipped on the PR and only failed on the next post-merge run that touched root Cargo.lock (#5838's libc bump).

Followed up:

• fix(prqlc/deps): set chrono minimum to 0.4.40 for arrow-arith compatibility #5841 set the floor to chrono = "0.4.40" (the highest known transitive minimum) — keeps the looser-than-0.4.44 spirit while staying honest with downstream consumers.
• ci: trigger nightly path filter on any Cargo.toml change #5843 widens the nightly path filter to **/Cargo.toml so future member-crate dep changes surface this class of regression on the PR itself.

Nothing to do here — just leaving a breadcrumb for anyone revisiting.

This was written by Claude Code on behalf of @max-sixty