fix(appstore): Cache apps.json also on dev instances

[SystemKeeper]

Summary

Currently caching of appstore files is disabled as soon as we are running beta, daily or git channel. When we run the current master and open index.php/settings/apps/discover we hit

server/lib/private/App/AppStore/Fetcher/Fetcher.php

Lines 111 to 117 in 09fef8f

	/**
	* Returns the array with the entries on the appstore server
	*
	* @param bool [$allowUnstable] Allow unstable releases
	* @return array
	*/
	public function get($allowUnstable = false) {

already 5 times, resulting in 5 requests to apps.json from the app store.

While it makes sense to have a shorter cache time for dev instances, we shouldn't omit all caching. This PR adds a 15 min caching (instead of 60 min on production) to dev instances.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[SystemKeeper]

/backport! to stable30

[SystemKeeper]

/backport! to stable29

[SystemKeeper]

/backport! to stable28