fix: compilation errors due to build cache

[erikburt]

We have seen compilation errors multiple times. This is seemingly due to a weird edge case with build caches.

Examples:

• Restore build cache from develop: https://github.com/smartcontractkit/chainlink/actions/runs/13294343412/job/37122743862?pr=16360
  • With debug info showing that the module exists in the module cache
• Full cache bust (no compilation errors): https://github.com/smartcontractkit/chainlink/actions/runs/13293709210/job/37120428599?pr=16360

Solution

After coming across golang/go#69179. Seems like its the module index that is not being handled properly in certain cases when restoring the build cache.

• "I don't think disabling the module index should cause any correctness problems."

   export GODEBUG=goindex=0
  

Further Notes

From what I understand the build cache uses a module index to resolve modules during the build step. If that module index doesn't correspond to the go module cache properly (ie. a go.mod was updated after the build cache was populated) then the index may map improperly causing compilation errors.

The two solutions I see for this is:

1. This change - disabling the go module index
2. Restricting build cache restores
   1. The build cache can restore from caches that are keyed with previous hashes of the go.mod, we could restrict it so it cannot do that. I'm guessing this would also fix the issue.

[cl-sonarqube-production]

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