fix: fix base url for preview command

[frank-zsy]

Description

This PR will fix the base URL config in the /preview command.

The config will read the env first to set the base URL, so in the action, we set the env to pull_${pull_number}, so the deployed website will use this as the base URL which not break all the relative links in the website.

Resolved issues

Closes #26

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[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/

[frank-zsy]

/preview

[github-actions[bot]]

🪧The website for this PR is deployed at https://open-digger.cn/pull_27/