Enable color diagnostics on circleci.

[iphydf]

See ninja-build/ninja#814

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This change is 

[sudden6]

Reviewed 2 of 2 files at r1.
Reviewable status: 0 of 1 approvals obtained

[codecov]

Codecov Report

Merging #1102 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1102     +/-   ##
========================================
- Coverage    82.6%   82.4%   -0.3%     
========================================
  Files          80      80             
  Lines       14316   14316             
========================================
- Hits        11834   11801     -33     
- Misses       2482    2515     +33

Impacted Files	Coverage Δ
toxcore/crypto_core_test.cc	90% <0%> (-10%)	⬇️
auto_tests/toxav_many_test.c	92.5% <0%> (-4.1%)	⬇️
auto_tests/toxav_basic_test.c	82.7% <0%> (-2.2%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-2.1%)	⬇️
toxav/toxav.c	68.1% <0%> (-1.5%)	⬇️
toxav/msi.c	65.6% <0%> (-1.2%)	⬇️
toxcore/onion_client.c	95% <0%> (-0.4%)	⬇️
toxcore/Messenger.c	85.7% <0%> (-0.3%)	⬇️
toxcore/net_crypto.c	93.6% <0%> (ø)	⬆️
auto_tests/conference_simple_test.c	96.8% <0%> (+3.1%)	⬆️

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