Fixes a issue in tryFindProgram where it would fail if the PATH environment variable contained relative paths, due to its incorrect assumption that the full_path argument is always absolute path.

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/30619
Co-authored-by: hixuyuming <hixuyuming@gmail.com>
Co-committed-by: hixuyuming <hixuyuming@gmail.com>