Scripts: opam update is done to late - it must be done before switch creation

[MSoegtropIMC]

@lennartberinger reported via email that the scripts didn't work for him on Ubuntu 22.04 failing with:

====================== JUST COQ OR COMPLETE PLATFORM ? =======================
Install full (f), extended (x), base (b) or IDE (i)? (f/x/b/i/c=cancel) x

(7): Coq 8.18.0 (released Sep 2023) with the preliminary package pick from Sep 2023
========================= SELECT PACKAGE_PICK VERSION ==========================
Select package list (number in 1..13, c=cancel) 7

=============================== PARALLEL BUILD ===============================
Build opam packages parallel (p) or sequential (s)? (p/s/c=cancel) p

Number of parallel make jobs (number in 1..16, c=cancel) 16

================================== COMPCERT ==================================
Install non open source SW CompCert (y) or (n)? (y/n/c=cancel) y

=============================== LARGE PACKAGES ===============================
Include (i) exclude (e) or select (s) large packages? (i/e/s/c=cancel) e

===== CHECKING VERSION OF INSTALLED OPAM =====
Found opam 2.1.3 - good!
/usr/local/bin/opam
===== opam already initialized =====
===== CREATE OPAM SWITCH =====
[CP.2023.03.0.patch_ocaml] Initialised
[CP.2023.03.0.patch_coq-released] Initialised
[CP.2023.03.0.patch_coq-dev] Initialised
[coq-released] synchronised from https://coq.inria.fr/opam/released
[coq-core-dev] synchronised from https://coq.inria.fr/opam/core-dev
[coq-extra-dev] synchronised from https://coq.inria.fr/opam/extra-dev

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml-variants" {= "4.14.1+options"} "ocaml-option-flambda"]
[ERROR] Could not determine which packages to install for this switch:
 * Missing dependency:
   - ocaml-variants > 5.1.1~
   no matching version


Switch initialisation failed: clean up? ('n' will leave the switch partially installed) [Y/n] y

According to Lennart an opam update fixed this. The scripts actually do a full opam update, but only after switch creation. This should be changed. The coq repos are update in the trascript above when they are added, but the main opam repo isn't. Without a switch one probably has to name the repos one wants to update.