testing: fix construction of the testing artifacts path

[ansel1]

The existing implementation constructs a path which starts with a forward
slash, which is then immediately rejected by filepath.Localize() as
invalid. This change simple removes the first forward slash.

Fixes #77763

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Patch Set 1:

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

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-02-24T23:57:06Z","revision":"89aecdd3d02b7e27481ac50e3e9d4c6f0dc7fa0f"}

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Patch Set 1:

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Patch Set 1: -Commit-Queue

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This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-amd64_avx512 has failed with summary (view all results):

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• go on master (change 748581)

To reproduce, try gomote repro 8688940709658523153.

Additional links for debugging:

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Patch Set 2:

(1 comment)

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