Add Cirrus CI configuration.

[iphydf]

This CI can run Bazel tests, because it supports IPv6. This is nice,
because now we can run IPv6 tests on every PR.

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This change is 

[codecov]

Codecov Report

Merging #1198 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1198     +/-   ##
========================================
- Coverage      83%   82.7%   -0.3%     
========================================
  Files          82      82             
  Lines       14646   14646             
========================================
- Hits        12163   12124     -39     
- Misses       2483    2522     +39

Impacted Files	Coverage Δ
auto_tests/network_test.c	90.3% <0%> (-9.7%)	⬇️
toxav/msi.c	63.9% <0%> (-2.8%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-1.2%)	⬇️
toxcore/net_crypto.c	93.6% <0%> (-1.2%)	⬇️
toxav/toxav.c	67.7% <0%> (-1%)	⬇️
toxcore/DHT.c	76.6% <0%> (-0.1%)	⬇️
toxcore/onion_client.c	95.8% <0%> (ø)	⬆️
toxcore/Messenger.c	86.7% <0%> (+0.1%)	⬆️

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