Specify python3 version in apt install commands

The default python3 is python 3.10 in debian:testing but this python version
has issues with installing meson and is ahead of gLinux. Specify 3.9 to
keep the version in CI the same as developers workstation.

Change-Id: Ibe2575c214499147134f82276964a34ae9856e6c

install-prereqs.sh

diff --git a/install-prereqs.sh b/install-prereqs.sh
index d44c388..99986d4 100755
--- a/install-prereqs.sh
+++ b/install-prereqs.sh
@@ -75,8 +75,8 @@
     pv
     python-is-python3
     python3-protobuf
-    python3
-    python3-dev
+    python3.9
+    python3.9-dev
     python3-pip
     rsync
     srecord