runtime: fix tests when running with -msan

[mauri870]

Currently go test -msan runtime always fails on loong64, arm64
and amd64. Changed the variable name to sz to match the
function prototypes.

For #64256

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Message from Keith Randall:

Patch Set 3: Auto-Submit+1 Code-Review+2 Commit-Queue+1

(1 comment)

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Message from Go LUCI:

Patch Set 3:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-01-09T00:52:50Z","revision":"4cf6e089808de2ea3b5090e543ffc5e9e4fec4cd"}

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Message from Keith Randall:

Patch Set 3: Code-Review+1

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Message from Keith Randall:

Patch Set 3: -Commit-Queue

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Patch Set 3:

This CL has passed the run

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Patch Set 3: LUCI-TryBot-Result+1

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This PR is being closed because golang.org/cl/547696 has been merged.