[Merged by Bors] - fix(tactic/interval_cases): instantiate metavars

[eric-wieser]

The test included in this commit fails without this change.

This does not need forward-porting, the test already passes in Lean 4.

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[bors[bot]]

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[eric-wieser]

bors merge

[bors[bot]]

Build failed:

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[eric-wieser]

bors merge

[bors[bot]]

Pull request successfully merged into master.

Build succeeded!

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