Add workaround for conan outage.

[kevinbackhouse]

Our CI stopped working due to a conan outage. According to this comment, the workaround is to add this to the CI scripts:

conan config install https://github.com/conan-io/conanclientcert.git

[kevinbackhouse]

According to this comment, this workaround needs to be added permanently.

[codecov]

Codecov Report

Merging #1939 (a65af1e) into 0.27-maintenance (a0ffcb8) will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           0.27-maintenance    #1939   +/-   ##
=================================================
  Coverage             46.32%   46.32%           
=================================================
  Files                   146      146           
  Lines                 22950    22950           
  Branches              11781    11781           
=================================================
  Hits                  10632    10632           
  Misses                 6702     6702           
  Partials               5616     5616           

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[clanmills]

Thanks for figuring this out, Kev. Software is slippery stuff.