'Expiration time' claim ('exp') must be a numeric value representing the future time at which the assertion expires

**PHP version**: 8.0.11

**Description**
I (occasionally) get the following error when attempting to authenticate as an app
```
'Expiration time' claim ('exp') must be a numeric value representing the future time at which the assertion expires
```

**Stacktrace**
```
#0 /var/task/vendor/php-http/httplug/src/Promise/HttpFulfilledPromise.php(31): Github\HttpClient\Plugin\GithubExceptionThrower->Github\HttpClient\Plugin\{closure}(Object(GuzzleHttp\Psr7\Response))
#1 /var/task/vendor/knplabs/github-api/lib/Github/HttpClient/Plugin/GithubExceptionThrower.php(124): Http\Client\Promise\HttpFulfilledPromise->then(Object(Closure))
#2 /var/task/vendor/php-http/client-common/src/PluginChain.php(48): Github\HttpClient\Plugin\GithubExceptionThrower->handleRequest(Object(Nyholm\Psr7\Request), Object(Closure), Object(Http\Client\Common\PluginChain))
#3 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(106): Http\Client\Common\PluginChain->Http\Client\Common\{closure}(Object(Nyholm\Psr7\Request))
#4 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(173): Http\Client\Common\Plugin\RetryPlugin->handleRequest(Object(Nyholm\Psr7\Request), Object(Closure), Object(Http\Client\Common\PluginChain))
#5 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(144): Http\Client\Common\Plugin\RetryPlugin->retry(Object(Nyholm\Psr7\Request), Object(Closure), Object(Http\Client\Common\PluginChain), '000000005952042...', 1000000)
#6 /var/task/vendor/php-http/httplug/src/Promise/HttpRejectedPromise.php(30): Http\Client\Common\Plugin\RetryPlugin->Http\Client\Common\Plugin\{closure}(Object(Github\Exception\RuntimeException))
#7 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(145): Http\Client\Promise\HttpRejectedPromise->then(Object(Closure), Object(Closure))
#8 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(173): Http\Client\Common\Plugin\RetryPlugin->handleRequest(Object(Nyholm\Psr7\Request), Object(Closure), Object(Http\Client\Common\PluginChain))
#9 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(144): Http\Client\Common\Plugin\RetryPlugin->retry(Object(Nyholm\Psr7\Request), Object(Closure), Object(Http\Client\Common\PluginChain), '000000005952042...', 500000)
#10 /var/task/vendor/php-http/httplug/src/Promise/HttpRejectedPromise.php(30): Http\Client\Common\Plugin\RetryPlugin->Http\Client\Common\Plugin\{closure}(Object(Github\Exception\RuntimeException))
#11 /var/task/vendor/php-http/client-common/src/Plugin/RetryPlugin.php(145): Http\Client\Promise\HttpRejectedPromise->then(Object(Closure), Object(Closure))
#12 /var/task/vendor/php-http/client-common/src/PluginChain.php(48): Http\Client\Common\Plugin\RetryPlugin->handleRequest(Object(Nyholm\Psr7\Request), Object(Closure), Object(Http\Client\Common\PluginChain))
#13 /var/task/vendor/php-http/client-common/src/PluginChain.php(63): Http\Client\Common\PluginChain->Http\Client\Common\{closure}(Object(Nyholm\Psr7\Request))
#14 /var/task/vendor/php-http/client-common/src/PluginClient.php(90): Http\Client\Common\PluginChain->__invoke(Object(Nyholm\Psr7\Request))
#15 /var/task/vendor/php-http/client-common/src/HttpMethodsClient.php(148): Http\Client\Common\PluginClient->sendRequest(Object(Nyholm\Psr7\Request))
#16 /var/task/vendor/php-http/client-common/src/HttpMethodsClient.php(108): Http\Client\Common\HttpMethodsClient->sendRequest(Object(Nyholm\Psr7\Request))
#17 /var/task/vendor/php-http/client-common/src/HttpMethodsClient.php(70): Http\Client\Common\HttpMethodsClient->send('POST', '/app/installati...', Array, NULL)
#18 /var/task/vendor/knplabs/github-api/lib/Github/Api/AbstractApi.php(145): Http\Client\Common\HttpMethodsClient->post('/app/installati...', Array, NULL)
#19 /var/task/vendor/knplabs/github-api/lib/Github/Api/AcceptHeaderTrait.php(34): Github\Api\AbstractApi->postRaw('/app/installati...', NULL, Array)
#20 /var/task/vendor/knplabs/github-api/lib/Github/Api/AbstractApi.php(127): Github\Api\Apps->postRaw('/app/installati...', NULL, Array)
#21 /var/task/vendor/knplabs/github-api/lib/Github/Api/AcceptHeaderTrait.php(29): Github\Api\AbstractApi->post('/app/installati...', Array, Array)
#22 /var/task/vendor/knplabs/github-api/lib/Github/Api/Apps.php(39): Github\Api\Apps->post('/app/installati...', Array)
#23 /var/task/app/Services/GithubService.php(29): Github\Api\Apps->createInstallationToken(******)
#24 /var/task/app/Jobs/ChannelReviewRequested.php(165): App\Services\GithubService->getConnection('******, ******)
```

Start at Github\Api\Apps::createInstallationToken in lib/Github/Api/Apps.php and follow the call from app/Services/GithubService.php line 29. Reproduce the app-authentication request and inspect how the expiration claim is produced; done means the request no longer intermittently returns this validation error and the behavior is verified.

索引モデルが issue の本文から書いたものです。