Switch to GitHub Actions.

[igorpeshansky]

Compared the configure output from Travis and the new GitHub actions. Most of the differences are in favor of GitHub actions (more stuff available in the environment and on Ubuntu Bionic), with the exception of:

• libi2c-dev . . . . . no (symbol i2c_smbus_read_i2c_block_data not found - have you installed libi2c-dev ?)
  • known breakage on Ubuntu Bionic
  • fixed by #2783, but that is not released until 5.10; we're on 5.8.1
• libsigrok . . . . . no (pkg-config could not find libsigrok)
  • expected on Ubuntu Bionic (collectd's configure looks for version <0.4, and Bionic comes with 0.5.0)
• libssl . . . . . . . no (Symbol 'SSL_library_init' not found)
  • expected on Ubuntu Bionic (that symbol is from libssl 1.0.x, and Bionic comes with 1.1.1)

Merging.