Fix empty tail partitions

[ychampion]

Summary

• Fix --partition bucket selection so later partitions do not go empty when earlier buckets round up.
• Add a regression covering the 11-run --feature-powerset --partition 5/5 case from --partition N/M doesn't distribute jobs properly #287.

Why

Fixes #287. With 11 generated runs and 5 partitions, the old ceil(total / partitions) bucket size assigned the first 9 runs to partitions 1-3, the final 2 runs to partition 4, and left partition 5 empty.

Validation

• /root/oss/cargo-hack/target/debug/cargo-hack hack check --feature-powerset --partition 5/5 --print-command-list from tests/fixtures/powerset_deduplication — printed the final two commands for partition 5 after the fix
• cargo test partition --test test -- --nocapture — passed
• cargo test --bin cargo-hack — passed
• cargo test --test test -- --skip multi_target --skip rust_version --skip version_range --skip version_range_failure — passed
• cargo clippy --all-targets --all-features -- -D warnings — passed
• cargo +nightly fmt --all -- --check — passed
• git diff --check — passed
• cargo test — did not complete locally because this host is out of disk while rustup downloads old toolchains for the rustup-dependent version/target tests

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