Merge #901: ci: Switch all Linux builds to Debian and more improvements

9361f360bb04156c7a0fa8f2664680b74d463ed5 ci: Select number of parallel make jobs depending on CI environment (Tim Ruffing)
28eccdf80641f71fada0ee4065c8127468162176 ci: Split output of logs into multiple sections (Tim Ruffing)
c7f754fe4d5e032fd150c4b9b985855e9fcaa521 ci: Run PRs on merge result instead of on the source branch (Tim Ruffing)
b994a8be3cf8ab0fc6a622980a9845bb82cc17db ci: Print information about binaries using "file" (Tim Ruffing)
f24e122d13db7061b1086ddfd21d3a1c5294213b ci: Switch all Linux builds to Debian (Tim Ruffing)

Pull request description:

  Best reviewed commit by commit

ACKs for top commit:
  jonasnick:
    ACK 9361f360bb04156c7a0fa8f2664680b74d463ed5
  sipa:
    utACK 9361f360bb04156c7a0fa8f2664680b74d463ed5

Tree-SHA512: fc754e8b57dc58058cebbf63a60ca76e08dbaefea1508ea27b7f962bce697c10033da3f57a35f731bc7cf3e210eb00e3b8985ae8b729d7bd83faee085b878b9c