Make better use of the new Octez Makefile

[pasqu4le]

Clarification and motivation

Recently some changes have been made to the build process and the Makefile in the upstream Octez repo, see tezos/#6957 in particular.

These changes have now been included in an Octez RC (16.0-rc1) and while the necessary changes have been made immediately here, we could take a deeper look and reconsider our builds.

For example, the OCTEZ_EXECUTABLES env var can now be used to specify what to build and, as mentioned in the Makefile:

```makefile
# Note that you can override the list of executables to build on the command-line, e.g.:
#
#     make OCTEZ_EXECUTABLES='octez-node octez-client'
#
# This is more efficient than 'make octez-node octez-client'
# because it only calls 'dune' once.

this can be more efficient.

Acceptance criteria

Our build process of Octez software is reconsidered and, if there is an opportunity to add options or efficiency it is adopted