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

[MacPorts]

#65183: rust: Portfile can fail to parse depending on build_arch
--------------------+-------------------------
 Reporter:  jmroot  |      Owner:  herbygillot
     Type:  defect  |     Status:  assigned
 Priority:  Normal  |  Milestone:
Component:  ports   |    Version:
 Keywords:          |       Port:  rust
--------------------+-------------------------
 For example, on macOS 12:
 {{{
 % port info rust build_arch=i386
 Error: Unable to open port: can't read "rustc_version": no such variable
 }}}
 It's fine for the port to only be installable for certain archs, but the
 Portfile should parse validly regardless of what platform or arch it is
 read on. This is probably what is preventing rust from showing up on
 ports.macports.org.

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