Fix main:* CI jobs appearing on develop branch pipelines

Replace $CI_DEFAULT_BRANCH with hardcoded "main" in all main:* job
rules. After the default branch was changed from main to develop,
these jobs incorrectly matched on develop pipelines because
$CI_DEFAULT_BRANCH now evaluates to "develop".

Jobs fixed:
- main:deploy-blog-mainnet
- main:deploy-wallet-mainnet
- main:deploy-blog-mirrornet
- main:deploy-wallet-mirrornet
- main:stop-blog-mainnet
- main:stop-wallet-mainnet
- main:stop-blog-mirrornet
- main:stop-wallet-mirrornet