util/docker/coreboot-sdk: Install an explicit version of GNAT

While Debian Sid provides GCC version 13, GNAT is still on version 12.
To keep them in sync, install GNAT 13 explicitly instead of the meta
package that is still referring to GNAT 12.

The coreboot toolchain including GNAT still compiles fine.

Change-Id: Ifb2b4c5fbaf3c0a8a78f6ebe244e2ccfec664b41
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77191
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

util/docker/coreboot-sdk/Dockerfile

@@ -35,7 +35,7 @@ RUN \
 		gawk \
 		gcc \
 		git \
-		gnat \
+		gnat-13 \
 		golang \
 		graphicsmagick-imagemagick-compat \
 		graphviz \