Fix pg18 CI failures: skip tools/osmium build for trixie in update.sh

[sanak]

Problem

All 18-3.6-* extra image builds have been failing since PR pgRouting#75 was merged.

The root cause is a missing template-level fix. PR pgRouting#75 commit 9fe515a
manually commented out the tools/osmium build lines in each
18-3.6-*/extra/Dockerfile, but did not update update.sh (or
extra/Dockerfile.template). As a result, the subsequent nightly
"Update hashes and versions" PR (pgRouting#76) ran make update, which
regenerated those files from the unchanged template — silently reverting
the comments and re-introducing the build failure.

The underlying incompatibility: libosmium2-dev on Debian Trixie (used
by pg18 images) has C++ API changes that break the tools/osmium build
in osm2pgrouting 3.0.0.
See: pgRouting/osm2pgrouting#323

Fix

Add a post-generation step in update.sh that comments out the
tools/osmium build block whenever the target Debian suite is trixie.
This ensures make update produces the correct output and the fix
survives future hash-update regenerations.

Also re-apply the comment-out to the five currently broken generated
files (18-3.6-{3.7,3.8,4.0,develop,main}/extra/Dockerfile).

Changes

• update.sh: after rendering extra/Dockerfile, comment out the
  tools/osmium build block when $suite = "trixie"
• 18-3.6-{3.7,3.8,4.0,develop,main}/extra/Dockerfile: re-generated
  with the osmium build skipped