steps: make SetProperty property renderable

[noc0lour]

there is no reason we can't have value a renderable, make setting properties more flexible.

• I have created a file in the master/buildbot/newsfragment directory (and read the README.txt in that directory)

[mention-bot]

@noc0lour, thanks for your PR! By analyzing the history of the files in this pull request, we identified @tardyp, @jvlomax and @sa2ajj to be potential reviewers.

[seankelly]

value was already renderable. I think you mean property.

[noc0lour]

Yeah, true, I'll fix that.

[codecov]

Codecov Report

Merging #3157 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3157      +/-   ##
==========================================
+ Coverage   88.06%   88.06%   +<.01%     
==========================================
  Files         314      314              
  Lines       33046    33047       +1     
==========================================
+ Hits        29101    29102       +1     
  Misses       3945     3945

Impacted Files	Coverage Δ
master/buildbot/steps/master.py	91.78% <100%> (ø)	⬆️
master/buildbot/steps/cmake.py	100% <0%> (ø)	⬆️

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[tardyp]

@noc0lour I've fixed the newsfragment myself

[noc0lour]

Thanks. I had that lying around somewhere but forgot to push it, apparently

[tardyp]

well, it is just two clicks away using github UI.