Use script.config in all build scripts, less typing please

The information in script.config could be used by scripts such as build_release_candidate.sh to avoid retyping information.

For example these values could be stored there:
```

RELEASE=2.18.0
RC_NUM=1
USER_GITHUB_ID=udim
SIGNING_KEY=587B049C36DAAFE6

```

Imported from Jira [BEAM-9090](https://issues.apache.org/jira/browse/BEAM-9090). Original Jira may contain additional context.
Reported by: udim.

Start by reading script.config and build_release_candidate.sh, then identify the other build scripts that repeat release information. Trace the release-candidate build flow and verify the scripts consistently use values from script.config without requiring duplicated input.

Written by the indexing model from the issue text.