Use the merge commit as seed version

[goodov]

Turns out, actions/checkout always uses a merge pin in workflows. This is good, because we will always get the test seed merged with the latest seed available in main, but to correctly differentiate such seed, we should use the merge commit as the seed version.

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1167.
   Android: Set the command line to --variations-pr=1167 in debug menu, restart the browser.
   iOS: Set Variations PR to 1167 in Brave Core Switches debug menu, restart the browser.
2. Wait 5-10 seconds to fetch the seed.
3. Restart the browser to apply the seed.
4. Ensure Active Variations section at brave://version starts with the expected seed version (see below).

Seed Details

Parameter	Value
Version	pull/1167@5ed1b0e57862ee6f9e429feb81e9bdf7fbc16d35
Uploaded	Thu, 08 Aug 2024 17:07:49 GMT
PR commit	3551a34
Base commit	7a0c55b
Merge commit	5ed1b0e
Serial number	537591d510424899bd7c08bb3dd71eda