Fix: Replaced import logic in predef script for Mill projects

[lefou]

The $MILL_BIN_PLATFORM replacement is only supported when used with import $ivy.

Fix issues introduced in #2821

[codecov]

Codecov Report

Base: 82.28% // Head: 82.28% // No change to project coverage 👍

Coverage data is based on head (71040bf) compared to base (183b757).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2823   +/-   ##
=======================================
  Coverage   82.28%   82.28%           
=======================================
  Files         148      148           
  Lines        2840     2840           
  Branches      204      204           
=======================================
  Hits         2337     2337           
  Misses        503      503           

Impacted Files	Coverage Δ
...org/scalasteward/core/buildtool/mill/MillAlg.scala	78.94% <ø> (ø)

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