abuild-sudo: make error msg "Not a member of group abuild" more clear

Few days ago some user on IRC pointed out that the current error message:

    abuild-apk: Not a member of group abuild

is confusing. He was trying to build a package using abuild -r and
didn't know what this message means.

fixes #5408

abuild-sudo.c

@@ -74,8 +74,10 @@ int main(int argc, const char *argv[])
 	if (grent == NULL)
 		errx(1, "%s: Group not found", ABUILD_GROUP);
 
-	if (!is_in_group(grent->gr_gid))
-		errx(1, "Not a member of group %s\n", ABUILD_GROUP);
+	if (!is_in_group(grent->gr_gid)) {
+		errx(1, "User %s is not a member of group %s\n",
+			getlogin(), ABUILD_GROUP);
+	}
 
 	cmd = strrchr(argv[0], '-');
 	if (cmd == NULL)