testing: allow specify temp dir by GOTMPDIR environment variable

[aisk]

Allow change the default temp directory returned by t.TempDir()
by environment variable GOTMPDIR.

Fixes #61585

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Patch Set 1:

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Patch Set 2:

(1 comment)

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Patch Set 2:

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Message from t hepudds:

Patch Set 3: Commit-Queue+1

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Message from Go LUCI:

Patch Set 3:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-08-02T12:21:00Z","revision":"f4cfd23d114d4ba90b5a7bfa196f9d183596f7c6"}

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Message from Go LUCI:

Patch Set 3: LUCI-TryBot-Result-1

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Patch Set 3:

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Message from t hepudds:

Patch Set 4: Commit-Queue+1

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Patch Set 4:

Dry run: CV is trying the patch.

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Message from t hepudds:

Patch Set 4: -Commit-Queue

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Patch Set 4:

This CL has passed the run

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Patch Set 4: LUCI-TryBot-Result+1

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