chore: avoid repeated "Mismatched <foo> output output" in expectString error message

[euprunin]

Example issue:

✖ [469/477] Building Manual.BuildTools.Elan
error: reference-manual/Manual/BuildTools/Elan.lean:226:0: Mismatched 'elan --help' output output:
                                                                                    ^^^^^^^^^^^^^

main branch:

Manual/Meta/ExpectString.lean:    logErrorAt expected m!"Mismatched {what} output:\n{Diff.linesToString diff}"
…
Manual/Meta/ElanCheck.lean:    discard <| expectString "'elan --help' output" str elanOutput (useLine := useLine) (preEq := String.trimRight)
Manual/Meta/ExpectString.lean:def expectString (what : String) (expected : StrLit) (actual : String)
Manual/Meta/LakeCheck.lean:    discard <| expectString "'lake --help' output" str lakeOutput (useLine := useLine)
Manual/Meta/LakeToml.lean:        discard <| expectString "elaborated configuration" expectedStr v (useLine := (·.any (!·.isWhitespace)))

This branch:

Manual/Meta/ExpectString.lean:    logErrorAt expected m!"Mismatched {what}:\n{Diff.linesToString diff}"
…
Manual/Meta/ElanCheck.lean:    discard <| expectString "'elan --help' output" str elanOutput (useLine := useLine) (preEq := String.trimRight)
Manual/Meta/ExpectString.lean:def expectString (what : String) (expected : StrLit) (actual : String)
Manual/Meta/LakeCheck.lean:    discard <| expectString "'lake --help' output" str lakeOutput (useLine := useLine)
Manual/Meta/LakeToml.lean:        discard <| expectString "elaborated configuration output" expectedStr v (useLine := (·.any (!·.isWhitespace)))

[github-actions]

Preview for this PR is ready! 🎉 (also as a proofreading version). built with commit af665fd.

[david-christiansen]

Thanks!