[MacPorts] #65183: rust: Portfile can fail to parse depending on build_arch

[MacPorts]

#65183: rust: Portfile can fail to parse depending on build_arch
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  Reporter:  jmroot  |      Owner:  MarcusCalhoun-Lopez
      Type:  defect  |     Status:  assigned
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:  rust    |
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Comment (by jmroot):

 An easy way to test is to run `port info rust build_arch=`. The fix looks
 good, thanks.

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Ticket URL: <https://trac.macports.org/ticket/65183#comment:5>
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