Make PR builder run on demand

[pavinduLakshan]

Purpose

$subject

Currently the identity-apps PR builder is triggered for each and every commit pushed to an open PR. This may cause a lot of workflow runs when there are multiple PRs. Since identity-apps receives frequent pull requests, it'd be better to make the pr builder run on demand so that we can avoid hitting concurrent build limits.

Related Issues

• N/A

Related PRs

• N/A

Checklist

• e2e cypress tests locally verified. (for internal contributers)
• Manual test round performed and verified.
• UX/UI review done on the final implementation.
• Documentation provided. (Add links if there are any)
• Relevant backend changes deployed and verified
• Unit tests provided. (Add links if there are any)
• Integration tests provided. (Add links if there are any)

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines
• Ran FindSecurityBugs plugin and verified report
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets