Ensures `Makefile` uses env settings

[dmalan]

[rongxin-liu]

Before merging, one concern is that if folks run check50 locally on their computer, they probably won't have LDLIBS and CFLAGS set in their env settings.

And do we want to use this approach for other checks (using env settings for Makefile)? I guess the motivation here is that we don't need to update each Makefile for each check when we make changes to CFLAGS and LDLIBS in cs50/cli?