os: optimise cgo clearenv

[cyphar]

For programs with very large environments, calling unsetenv(3) for each
environment variable can be fairly expensive because of CGo overhead,
but clearenv(3) is much faster. The only thing we have to track is
whether GODEBUG is being unset by the operation, which can be done very
quickly without resorting to doing unsetenv(3) for every variable.

This change makes os.Clearenv() ~30% faster when run an environment with
1000 environment variable set (and gets better with more environment
variables):

cpu: AMD Ryzen 7 7840U w/ Radeon  780M Graphics
       │   old.txt   │               new.txt               │
       │   sec/op    │   sec/op     vs base                │
Env-16   213.6µ ± 1%   185.1µ ± 3%  -13.32% (p=0.000 n=10)

(The time taken to set up 1000 variables in this benchmark is 117.2µs,
so while the naive timing is 13% the speedup for os.Clearenv() itself is
from 96.4µs to 67.9µs which is ~30%.)

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Patch Set 1:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 1:

(3 comments)

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[gopherbot]

Message from Aleksa Sarai:

Patch Set 1:

(4 comments)

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Message from Ian Lance Taylor:

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":"2024-12-05T04:30:15Z","revision":"19cbf5e28344a2b4c3819f31ce0d98cfad575db6"}

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Message from Ian Lance Taylor:

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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Patch Set 3:

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Message from Kirill Kolyshkin:

Patch Set 3: Code-Review+1

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