os/exec: avoid calling LookPath in cmd.Start for resolved paths

[qiulaidongfeng]

Follow up on CL 511458, see https://go-review.googlesource.com/c/go/+/511458/2..4/src/cmd/go/main.go#b270 .

For #36768.

Change-Id: Icc2a4dbb1219b1d69dd10a900478957b0e975847

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Message from Ian Lance Taylor:

Patch Set 2: Run-TryBot+1

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Patch Set 2:

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Patch Set 2:

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Patch Set 2: TryBot-Result-1

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Patch Set 3:

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Message from Bryan Mills:

Patch Set 3:

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Patch Set 5:

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Message from Bryan Mills:

Patch Set 5: Auto-Submit+1 Code-Review+2 Run-TryBot+1

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Patch Set 5:

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Patch Set 5: TryBot-Result+1

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This PR is being closed because golang.org/cl/512155 has been merged.