feedgnuplot: fix big sur build

[chenrui333]

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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
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As discussed with #66370 (comment) Big Sur's system perl changed how PREFIX= is treated as an install destination. The more explicit way to control MakeMaker is to set INSTALL_BASE instead, which should result in consistent behavior between different OS/X versions.

cc @mitchblank

[mitchblank]

I don't understand why you needed to make a new PR when #66370 is already open? Looks like mostly the same changes..

[chenrui333]

i am sorry, that is my bad, I did not see the change. Closing.

[chenrui333]

move the changes to there, thanks @mitchblank!