Issue being fixed or feature implemented

Make CI builds faster.

We have this https://github.com/dashpay/dash/blob/develop/ci/dash/build_src.sh#L18 but by that time CCACHE_SIZE is set to 100M via https://github.com/dashpay/dash/blob/develop/ci/test/00_setup_env.sh#L51 already.

What was done?

set CCACHE_SIZE variable in .gitlab-ci.yml, drop confusing and useless lines from ci/dash/build_src.sh (CCACHE_SIZE and CCACHE_COMPRESS defaults are handled by ci/test/00_setup_env.sh)

How Has This Been Tested?

I set CCACHE_SIZE to 400M yesterday via Gitlab UI to test it.

results:
before, trivial doc change, 100M: https://gitlab.com/dashpay/dash/-/jobs/4931233566#L59 (~17 minutes)
after, trivial code change, 400M: https://gitlab.com/dashpay/dash/-/jobs/4935764148#L80 (~12 minutes)

Removed it from Gitlab UI now but it should still say Set cache size limit to 400.0 MB in logs for this PR.

Breaking Changes

n/a

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)