Allow all tags when specifying the agent on a step

[uniquefine]

Currently only the queue tag can be used when specifying a step's agents selector.
This PR allows the user to target agents using any tag.

We hit this problem because we use tags to select agents for a pipeline step.

[codecov-commenter]

Codecov Report

Merging #85 (6421eda) into master (40e837e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #85   +/-   ##
=======================================
  Coverage   78.98%   78.98%           
=======================================
  Files           4        4           
  Lines         157      157           
=======================================
  Hits          124      124           
  Misses         21       21           
  Partials       12       12           

Impacted Files	Coverage Δ
plugin.go	93.22% <ø> (ø)

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