oauth2: use strings.Builder instead of bytes.Buffer

[ash2k]

The former does not make a copy of the accumulated buffer
to produce a string.

WriteByte() is faster than WriteRune() and we are not
appending non-ASCII here.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Mikhail Mazurskiy:

Patch Set 1:

(1 comment)

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[octavio12345300]

List

[octavio12345300]

List

[gopherbot]

Message from Sean Liao:

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

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Message from Go LUCI:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-09-13T09:35:46Z","revision":"0ee0ccabf8d029d70509cab743c9c52c39a78a59"}

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Message from Sean Liao:

Patch Set 2: -Commit-Queue

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Patch Set 2:

This CL has passed the run

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Message from Go LUCI:

Patch Set 2: LUCI-TryBot-Result+1

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