Allow for more clock drift when generating the JWT

In my tests, sometimes even though the 10 minutes maximum expire time
worked when searching for the comment, it still was possible for the
subsequent request (appending to the comment) to fail with:

	'Expiration time' claim ('exp') is too far in the future

Let's work around that by avoiding the full 10 minutes expiry and going
for 9 instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

GitForWindowsHelper/github-api-request-as-app.js

@@ -9,8 +9,8 @@ module.exports = async (context, requestMethod, requestPath, body) => {
     const payload = {
         // issued at time, 60 seconds in the past to allow for clock drift
         iat: now - 60,
-        // JWT expiration time (10 minute maximum)
-        exp: now + (10 * 60),
+        // JWT expiration time (10 minute maximum, use 9 to allow for clock drift)
+        exp: now + (9 * 60),
         // GitHub App's identifier
         iss: process.env['GITHUB_APP_ID']
     }