Update ImageMagick in CI environment to support HEIF/HEIC test cases

Steps to reproduce

1. Submit a pullrequest like https://github.com/nextcloud/server/pull/10526 which requires a recent version of ImageMagick convert tool
2. CI test case fails: https://drone.nextcloud.com/nextcloud/server/9604

Expected behaviour

Test case should report green.

Actual behaviour

Due to the version of ImageMagick which lacks support to read HEIF/HEIC files (the new Apple file format for iPhones) in the CI environment, the test case fails (see log below).

Server configuration

Running the tests on macOS with ImageMagick 7.0.8-9 installed through Homebrew works:
brew install --with-libheif imagemagick

Operating system:
macOS 10.13.6

Web server:
embedded php webserver

Database:
embedded sqlite

PHP version:
7.2

Nextcloud version: (see Nextcloud admin page)
github master checkout

Updated from an older Nextcloud/ownCloud or fresh install:
new install

Where did you install Nextcloud from:
github

Logs

CI log (data/nextcloud.log)

1) Test\Preview\HEICTest::testGetThumbnail with data set #0 (-85, -61)
Failed asserting that false is not equal to false.

/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:141
/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:103

2) Test\Preview\HEICTest::testGetThumbnail with data set #1 (34, 26)
Failed asserting that false is not equal to false.

/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:141
/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:103

3) Test\Preview\HEICTest::testGetThumbnail with data set #2 (-76, 55)
Failed asserting that false is not equal to false.

/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:141
/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:103

4) Test\Preview\HEICTest::testGetThumbnail with data set #3 (75, -84)
Failed asserting that false is not equal to false.

/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:141
/drone/src/github.com/nextcloud/server/tests/lib/Preview/Provider.php:103

Start with Test\Preview\HEICTest and the failing assertions in tests/lib/Preview/Provider.php, then inspect the CI environment configuration in this docker-ci repository. Update the ImageMagick setup so it can read HEIF/HEIC files, and confirm that the referenced preview tests pass in CI.