ci: dynamically set thread-count

Despite what GitHub states in their documentation, the runners feature a
different core count. Automatically set the build thread-count by the
amount of available CPU cores.

Link: https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources



Signed-off-by: David Bauer <mail@david-bauer.net>