KNITROAMPL: use \$KNITRODIR/knitroampl/knitroampl for executable

[yemine0x01]

Fixes # .

NA

Summary/Motivation:

Some KNITRO users install it as a standalone package (not via the Python knitro module) and rely on the KNITRODIR environment variable to locate their installation. Previously, _default_executable() only checked the knitro Python package path first, then fell back to a plain PATH search, meaning users with a KNITRODIR-based installation but no Python package would only succeed if knitroampl happened to be on their PATH.

This PR adds an explicit lookup for $KNITRODIR/knitroampl/knitroampl as the highest-priority candidate, reflecting the standard layout of a standalone KNITRO installation.

Changes proposed in this PR:

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