cmd/go: honor -fullpath for compiler diagnostics

[amirHdev]

Promote -fullpath into shared cmd/go build flag handling so it affects
cmd/go error formatting as well as forwarded test binary output.
When -fullpath is set avoid shortening file names in command output
and package-load errors. add script coverage for compiler diagnostics
with "go test -C ... -fullpath".
Fixes #60451

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Michael Matloob:

Patch Set 1: Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-04-06T19:25:10Z","revision":"63dac143291979c3400cd4fd803c0cf99587f54c"}

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Message from Michael Matloob:

Patch Set 1: -Commit-Queue

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Message from Go LUCI:

Patch Set 1:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-windows-amd64-longtest has failed with summary (view all results):

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• go on master (change 762961)

To reproduce, try gomote repro 8685243342001371073.

Additional links for debugging:

• go tool dist test -json output [shard 1]

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Patch Set 1: LUCI-TryBot-Result-1

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Message from Michael Matloob:

Patch Set 1:

(1 comment)

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