Remove RSpec warnings from `spec/models/ci/build_spec.rb`

Resolves these warnings:

......................WARNING: Using `expect { }.not_to raise_error(SpecificErrorClass)` risks false positives, since literally any other error would cause the expectation to pass, including those raised by Ruby (e.g. NoMethodError, NameError and ArgumentError), meaning the code you are intending to test may not even get reached. Instead consider using `expect { }.not_to raise_error` or `expect { }.to raise_error(DifferentSpecificErrorClass)`. This message can be suppressed by setting: `RSpec::Expectations.configuration.on_potential_false_positives = :nothing`. Called from /Users/rspeicher/Code/gitlab/gdk-ce/gitlab-ce/spec/models/ci/build_spec.rb:1961:in `block (5 levels) in <top (required)>'.
........................................................................................................................................................................................................user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
............user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
.user is a NilClass - rspec-set works with ActiveRecord models only
....

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248 examples, 0 failures

Randomized with seed 1189

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