feat: pass all LOG_* environment variables to the Renovate container

[ruzickap]

Description

Renovate supports several logging-related environment variables that are loaded
before configuration parsing (docs):

• LOG_CONTEXT
• LOG_FILE
• LOG_FILE_FORMAT
• LOG_FILE_LEVEL
• LOG_FORMAT
• LOG_LEVEL

Currently the default env-regex only allows LOG_LEVEL through to the
Docker container. Users who want to use LOG_FILE (to enable file logging)
must override the entire env-regex input just to add one variable.

This PR changes the default pattern from LOG_LEVEL to LOG_\w+ so that
all Renovate logging variables are forwarded without requiring users to
customize env-regex.

Changes

• src/input.ts — default regex: LOG_LEVEL → LOG_\w+
• action.yml — updated description of the default value
• README.md — updated the env-regex example

Motivation

When using LOG_FILE with docker-volumes: /tmp:/tmp to write Renovate
logs and upload them as artifacts, the log file is never created because
LOG_FILE is silently dropped by the action's env filtering.

Example workflow that fails without this change:

- name: Self-hosted Renovate
  uses: renovatebot/github-action@v46.1.14
  with:
    configurationFile: .github/renovate-global.json5
    token: ${{ steps.app-token.outputs.token }}
    docker-volumes: /tmp:/tmp
  env:
    LOG_FILE: /tmp/renovate-log.json

- name: Upload Renovate log
  if: always()
  uses: actions/upload-artifact@v4
  with:
    name: renovate-log
    path: /tmp/renovate-log.json

The workaround today is to specify a custom env-regex that includes
LOG_FILE, but this is non-obvious and error-prone since users expect
logging variables to work the same way LOG_LEVEL does.

[viceice]

you should use RENOVATE_LOG_ prefix for log variables

@jamietanna @secustor I think we discussed already to change the docs. maybe forgot to do that?