[CI] Fix SemVer check base commit (#5361)

After seeing some cases of reported changes that did not happen by the
merge request proposer (like
https://github.com/paritytech/polkadot-sdk/pull/5339), it became clear
that [this](https://github.com/orgs/community/discussions/59677) is
probably the issue.
The base commit of the SemVer check CI is currently using the *latest*
master commit, instead of the master commit at the time when the MR was
created.

Trying to get the correct base commit now. For this to be debugged, i
have to wait until another MR is merged into master.

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>