CI: Coverity tweaks

CI: Coverity tweaks

 * Use workflow_dispatch to allow manual creation of the job.
 * Use parallel builds; the workers have two vCPUs.  Also, use the build-*
   targets rather than the ones which expand to dist-*.
 * Shrink the dependency list further.  build-essential covers make and gcc,
   while bridge-utils and iproute2 are runtime dependencies not build
   dependencies.  Alter bzip2 to libbz2-dev.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>