Move the tldr.zip missing warning from runtime (uudoc.rs) to compile time (build.rs). This ensures the warning is printed only once during the build, instead of 100+ times when make calls uudoc separately for each utility.

The Problem

From issue #9940:

Calling this for all utils pollutes the build log. No need to show it 100+ times.

The previous OnceLock approach did not work because make install-manpages invokes uudoc manpage <utility> as a separate process for each utility. Since each process has its own memory, OnceLock resets every time.

The Fix

Check for docs/tldr.zip in build.rs and emit a cargo:warning= message at compile time:

// build.rs
if env::var("CARGO_FEATURE_UUDOC").is_ok() && !Path::new("docs/tldr.zip").exists() {
    println!("cargo:warning=No tldr archive found, so the documentation will not include examples.");
    println!("cargo:warning=To include examples, download the tldr archive:");
    println!("cargo:warning=  curl -L https://github.com/tldr-pages/tldr/releases/latest/download/tldr.zip -o docs/tldr.zip");
}

This prints the warning exactly once when cargo build --features uudoc runs, not during the 100+ runtime invocations.

Testing

Build prints the warning once:

$ cargo build --bin uudoc --features "uudoc feat_os_unix"
   Compiling coreutils v0.5.0 (/workspace)
warning: [email protected]: No tldr archive found, so the documentation will not include examples.
warning: [email protected]: To include examples, download the tldr archive:
warning: [email protected]:   curl -L https://github.com/tldr-pages/tldr/releases/latest/download/tldr.zip -o docs/tldr.zip
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 5.39s

Running uudoc manpage for multiple utilities (simulating make install-manpages) prints no warnings:

$ for util in ls cat cp mv rm mkdir rmdir echo head tail; do ./target/debug/uudoc manpage $util 2>&1 >/dev/null; done
(no output - no warnings printed!)

Fixes #9940