Only push to assets-compile-cache when in master

What does this MR do?

There are a huge number of files in the assets-compile-cache. This causes the zipping and uploading of cache files to take a long time. Almost doubling CI running time of assets-compile.

This MR prevents runaway growth of the cache by setting the cache policy to pull-push only in master and just pull otherwise.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team