cli: tweak how "is one file" predicate works

In effect, we switch from `path.is_file()` to `!path.is_dir()`. In cases
where process substitution is used, for example, the path can actually
have type "fifo" instead of "file." Even if it's a fifo, we want to
treat it as-if it were a file. The real key here is that we basically
always want to consider a lone argument as a file so long as we know it
isn't a directory. Because a directory is the only thing that will
causes us to (potentially) search more than one thing.

Fixes #2736

crates/cli/src/lib.rs

@@ -182,6 +182,7 @@ pub fn is_readable_stdin() -> bool {
         let file = File::from(fd);
         let Ok(md) = file.metadata() else { return false };
         let ft = md.file_type();
+        dbg!(&ft);
         ft.is_file() || ft.is_fifo() || ft.is_socket()
     }