ci: build.sh: Add a wrapper function over meson's setup

ci: build.sh: Add a wrapper function over meson's setup

The reason for this wrapper is that all job functions introduced in
future patches will refer to this one instead of open-coding the same
'meson setup' invocation N times. It also prevents 'setup' to be called
multiple times as some future job functions might actually do just that
in a transitive manner.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>