Use %cs format when getting the commit date

From the man page:

    %cs
        committer date, short format (YYYY-MM-DD)

This is identical to what we use, without having to specify a custom
date format for it.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

Makefile

@@ -17,7 +17,7 @@ all:
 	@exit 1
 else
 # Calculate version
-DATE=$(shell git show --format="%cd" --date="format:%Y-%m-%d" --no-patch --no-show-signature)
+DATE=$(shell git show --format="%cs" --no-patch --no-show-signature)
 REV=$(shell git describe --abbrev=7 --always --dirty)
 VERSION?=$(DATE)_$(REV)