Another attempt to fix docs.rs builds

[bavshin-f5]

Should fix the following error:

[INFO] [stderr]   --- stderr
[INFO] [stderr] 
[INFO] [stderr]   thread 'main' (15) panicked at build/main.rs:139:67:
[INFO] [stderr]   nginx-src build: Os { code: 30, kind: ReadOnlyFilesystem, message: "Read-only file system" }
[INFO] [stderr]   note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Copilot]

Pull Request Overview

This PR fixes docs.rs build failures by changing the cache directory location from CARGO_MANIFEST_DIR to OUT_DIR, which addresses a read-only file system error during documentation builds.

• Updates nginx-src dependency version from ~1.28.0 to ~1.28.1
• Refactors cache directory initialization from a function to a static lazy initialization using OUT_DIR
• Changes cache directory base from manifest directory to build output directory to avoid read-only filesystem issues

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
nginx-sys/Cargo.toml	Updates nginx-src dependency version to match new release
nginx-src/Cargo.toml	Bumps crate version to 1.28.1 while maintaining NGINX version compatibility
nginx-src/src/download.rs	Refactors cache directory handling to use OUT_DIR and static initialization

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[ensh63]

Looks reasonable.