run circleci as non-root user

[alexander-fenster]

It saves several minutes per CI task because it keeps recompiling grpc being unable to unpack it. Some details here.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov-io]

Codecov Report

Merging #24 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #24   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines           4      4           
=====================================
  Hits            4      4

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[alexander-fenster]

@lukesneeringer or @matthewayne - ping please :) This one will speed up CI a lot (we don't have any better solution so far).