Fix broken onboarding with a live account

[diegocurbelo]

Fixes #3320 (comment)

Changes proposed in this Pull Request:

This PR fixes the error introduced in #3320, where the transient was been overwritten when rendering 2 Connect OAuth URLs (Live and test).

Before this fix, after choosing (or creating) the account on the Stripe site, the redirect validation failed, and the merchant was redirected again to the onboarding screen:

Testing instructions

0. Disconnect from Stripe (if needed)
   
1. Navigate to WP-Admin > WooCommerce > Settings > Payments > Stripe
2. Click on the Create or connect an account, and complete the onboarding on the Stripe site (create or choose an account from the list)
   
3. Check that the redirect is successful, the plugin is enabled in live mode, and the email in the Account details is correct:
   

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• Covered with tests (or have a good reason not to test in the description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[Mayisha]

Thanks @diegocurbelo for fixing this 👍

In develop branch I can reproduce this in two scenarios-

• First scenario is when Stripe is disconnected and I click the Create or connect an account button on the onboarding screen. After choosing an account I am redirected again to the onboarding screen.
• Second scenario is when the test account is connected and I click the Create or connect an account button on the account connection modal. After choosing an account I am redirected to the Stripe settings page just fine, but the live account is still disconnected.

In this branch both of the above issues are fixed.
✅ Connecting to a live account from the onboarding page works.
✅ Connecting to a live account from the account connection modal when the test account is connected also works.