RetryCommands.ps1

function Main() {
    RetryCommand `
            -maxRetries 5 `
            -delayInSeconds 0 `
            -command { cmd /c $SIGNTOOL sign /v /debug /sm /t $TIMESTAMP_URL $files }

    RetryCommand `
        -maxRetries 5 `
        -delayInSeconds 3 `
        -command { . $INSTALLSHIELD $arguments }
}

function RetryCommand($maxRetries, $delayInSeconds, $command) {
    $textCommand = $ExecutionContext.InvokeCommand.ExpandString($command)
    LogImportant "Calling `n`t $textCommand"
    $failedTimes = 0
    do {
        & $command
        $commandExitCode = $lastExitCode
        $successful = $commandExitCode -eq 0
        if (-not $successful) { $failedTimes++ }

        $shouldRetry = -not $successful -and $failedTimes -le $maxRetries
        if ($shouldRetry) {
            $retriesLeft = $maxRetries - $failedTimes + 1
            LogImportant "FAILED!!! Command: $textCommand"
            LogImportant "We'll retry again after $delayInSeconds seconds ($retriesLeft/$maxRetries retries left)..."
            Start-Sleep -Seconds $delayInSeconds
        }
    } while ($shouldRetry)

    if (-not $successful) { Write-Error "FAILED!!! Command: $textCommand (exit code = $commandExitCode)" }
}

Main

Stuff to note: forcing string interpolation (variable substitution):

$ExecutionContext.InvokeCommand.ExpandString($command)

Other stuff: only $lastExitCode works in this case. $? won't work!