cmd/go: avoid when go.env contain GOTOOLCHAIN=local test fail

[qiulaidongfeng]

The test fail when $GOROOT/go.env contain GOTOOLCHAIN=local
because GOTOOLCHAIN=local is assumed to be a non-default value.
This CL fixed the test failure
by using go.env from the test as $GOROOT/go.env throughout the test.
Test have also been added to ensure that
when $GOROOT/go.env contain GOTOOLCHAIN=local,
GOTOOLCHAIN=local is not taken as a non-default value.

Fixes #67793

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from qiu laidongfeng2:

Patch Set 1: Commit-Queue+1

(1 comment)

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-06-04T13:03:29Z","revision":"30014baa14b82049c12e09a6df638c94bfdcfcc2"}

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Message from qiu laidongfeng2:

Patch Set 1: -Commit-Queue

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Patch Set 1:

This CL has passed the run

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Michael Matloob:

Patch Set 1:

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Message from qiu laidongfeng2:

Patch Set 3:

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Message from Sam Thanawalla:

Patch Set 3: Code-Review+1

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This PR is being closed because golang.org/cl/590196 has been merged.