fix ./mk dependencies build issue

the bug was actually caused by chkvars

add an escape for the quotes and bam. fixed.

without this, i got the following e.g.

For command: ./mk dependencies debian

Output:

./mk: 1: [: apt-get: unexpected operator
ERROR ./mk: pkg_add unset

Someone reported a similar issue with the Arch one,
which is also now fixed. This regression was caused
by the previous commit:

commit 0cf58c2273
Author: Leah Rowe <leah@libreboot.org>
Date:   Thu Jan 2 23:52:45 2025 +0000

    fix lbmk shellcheck errors

I forgot to escape the double quotes in an eval.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/lib.sh

@@ -41,7 +41,7 @@ chkvars()
 {
 	for var in "$@"; do
 		eval "[ -n "\${$var+x}" ] || \$err \"$var unset\""
-		eval "[ -n "\$$var" ] || \$err \"$var unset\""
+		eval "[ -n \"\$$var\" ] || \$err \"$var unset\""
 	done; return 0
 }