fix errors reported by go vet

[onkarvhanumante]

• As of now, the validate.sh script has been executed as part of the PR checks. However, there was an issue with how the go vet command was invoked in the script.

  prebid-server/validate.sh

  Lines 59 to 63 in 95c269f

  	if $VET; then
  	COMMAND="go vet"
  	echo "Running: $COMMAND"
  	`$COMMAND`
  	fi

  
  

• Currently, the go vet command is not checking all the files. This resulted to errors/ warnings not being captured during the PR checks.

• The solution is to modify the command to go vet ./... which will ensure that all files are thoroughly checked during the PR checks.

  % (go vet ./... 2>&1) | grep -v -E "composite literal uses unkeyed fields"
  # github.com/prebid/prebid-server/amp
  # github.com/prebid/prebid-server/adapters/adoppler
  # github.com/prebid/prebid-server/adapters/huaweiads
  adapters/huaweiads/huaweiads.go:905:14: result of fmt.Errorf call not used
  # github.com/prebid/prebid-server/adapters/rubicon
  adapters/rubicon/rubicon_test.go:293:9: GetRate passes lock by value: github.com/prebid/prebid- 
  server/adapters/rubicon.mockCurrencyConversion contains github.com/stretchr/testify/mock.Mock contains sync.Mutex
  adapters/rubicon/rubicon_test.go:298:9: GetRates passes lock by value: github.com/prebid/prebid- 
  server/adapters/rubicon.mockCurrencyConversion contains github.com/stretchr/testify/mock.Mock contains sync.Mutex
  # github.com/prebid/prebid-server/adapters/taboola
  adapters/taboola/taboola.go:256:13: result of fmt.Errorf call not used
  

• The plan here is to update the script to execute the go vet ./... command. However, before implementing this change, let's address any previously uncaught errors or warnings. PR makes changes to fix some errors or warnings reported by go vet

[SyntaxNode]

I'm fine with this change to make go vet happy, but the existing code is perfectly fine since it's just a mock and doesn't utilize the lock.