[Merged by Bors] - fix(tactic/linarith): instantiate metavariables in linarith

[eric-wieser]

This probably isn't exhaustive, but catches the really trivial case.

This doesn't appear to need forward-porting.

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[semorrison]

bors merge

[bors[bot]]

Pull request successfully merged into master.

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• Build mathlib
• Lint mathlib
• Lint style
• Run tests