fix(select): Fix invalid use of dropdown arrow mixin

[williamernest]

Fixes: #2636

[codecov-io]

Codecov Report

Merging #2637 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2637   +/-   ##
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  Coverage   98.48%   98.48%           
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  Files          98       98           
  Lines        4230     4230           
  Branches      537      537           
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  Hits         4166     4166           
  Misses         64       64

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[kfranqueiro]

Approving this since it fixes an issue... but I think we can simplify this whole situation.

Interestingly, I don't think the previous code was even generating a valid background-image value to begin with. It occurs to me that now this code is using on-surface, whereas the SCSS for the default arrow is hard-coding to black, which makes me wonder if we should actually be defaulting to on-surface and don't need to re-call the svg-bg mixin whatsoever? (The separate opacity rule seems to be doing all of the work.)