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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[jsiirola]

This looks reasonable; however, should the order in which paths are tested be reversed? That is, if the knitro module is available, we should probably use the knitroampl from it (for consistency) before falling back on KNITRODIR?

[yemine0x01]

Thanks for the review @jsiirola. I checked with the KNITRO dev team on the ordering, and they'd rather keep KNITRODIR first, ahead of the knitro module. Their reasoning is that if a user has set KNITRODIR, that's a deliberate pointer to the install they want used, so it should win. Does that line up with how you're thinking about it?

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.14%. Comparing base (1c7cda3) to head (ffd5b3b).

Files with missing lines	Patch %	Lines
pyomo/solvers/plugins/solvers/KNITROAMPL.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3979   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files         917      917           
  Lines      109091   109100    +9     
=======================================
+ Hits        98337    98346    +9     
  Misses      10754    10754           

Flag	Coverage Δ
builders	29.09% <71.42%> (+<0.01%)	⬆️
default	86.13% <85.71%> (?)
expensive	35.08% <85.71%> (?)
linux	87.66% <71.42%> (-1.98%)	⬇️
linux_other	87.66% <71.42%> (+<0.01%)	⬆️
oldsolvers	28.03% <71.42%> (+<0.01%)	⬆️
osx	83.11% <71.42%> (-0.01%)	⬇️
win	85.42% <71.42%> (-0.01%)	⬇️

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