Fix the CTP7 CI job

Description

The title says it all. It is damn time to fix this CI...

The changes are the following:

• The Linaro SDK is downloaded from a CMS GEM DAQ mirror (and installed on the gem904daq04 machine)
• The fixed and complete CTP7 sysroot is pushed on the CMS GEM DAQ website (and install on the gem904daq04 machine)
• The container is updated before running any job in order to avoid #162 (closed) with new updates
• The developers' instructions are updated to match the CI behavior (i.e. using the Linaro SDK for cross-compilation and the fixed CTP7 sysroot)

Related Issue

Closes #162 (closed)

How Has This Been Tested?

The CI passes. Developers' instructions on the gem904daq04 machine are working from a clean build.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.