fix(btls-sys): skip nonexistent link search directories

[18o]

Summary

Only emit cargo:rustc-link-search directives for directories that actually exist.

The BoringSSL build script currently emits several candidate library search paths. Depending on the generated CMake layout, directories such as build/lib and build/crypto may not exist because libcrypto.a and libssl.a are placed directly under build/.

These nonexistent paths are still passed to the linker as -L arguments.

Motivation

Rust 1.97 reports linker output by default, making the existing invalid search paths visible:

warning: linker stderr: unable to open library directory '.../build/lib': FileNotFound
warning: unable to open library directory '.../build/crypto': FileNotFound

This is reproducible when cross-compiling with cargo-zigbuild. It is not specific to Zig: any linker that reports nonexistent library
search directories may expose the same issue.

## Changes

Check each candidate path with Path::is_dir() before emitting its cargo:rustc-link-search directive.

This removes invalid linker arguments without changing the selected libraries or the existing platform-specific directory resolution.

## Verification

- Ran cargo test -p btls-sys
- Cross-compiled the mk_certs example for x86_64-unknown-linux-gnu
- Confirmed that the nonexistent library directory warnings are no longer emitted

[0x676e67]

nice job