fix: do not read ahead in STDIN

The Open Group Base Specifications Issue 8 IEEE Std 1003.1-2024 sh — shell, the
standard command language interpreter says:

> When the shell is using standard input and it invokes a command that also uses
> standard input, the shell shall ensure that the standard input file pointer
> points directly after the command it has read when the command begins
> execution. It shall not read ahead in such a manner that any characters
> intended to be read by the invoked command are consumed by the shell (whether
> interpreted by the shell or not) or that characters that are not read by the
> invoked command are not seen by the shell.

We used the default BUFFER_SIZE for get_next_line of 1024, which caused us to
read ahead farther than was allowed by the Open Group Base Specification.

Setting BUFFER_SIZE=1 ensures that we don't read too far ahead, since
get_next_line will always immediatly stop once a newline is found.

This is for me the simplest way to solve this issue.

Makefile

@@ -100,6 +100,7 @@ all: $(NAME)
 $(NAME): $(minishell_objs) $(LIBFT)
 	$(CC) $(CFLAGS) -o $@ $(minishell_objs) $(LINCLUDE) $(LDLIBS)
 
+$(LIBFT): CFLAGS+=-DBUFFER_SIZE=1
 $(LIBFT):
 	+$(MAKE) -C $(LIBFTDIR)