[Backport 6.2] ./github/workflows/add-label-when-promoted.yaml: Run auto-backport only on default branch

[scylladbbot]

In #21496 (comment)

scylladbbot force-pushed the backport/21459/to-6.1 branch from 414691c to 59a4ccd Compare 2 days ago

Backport automation is triggered by push but should also start from the master branch (or enterprise branches from Enterprise). We need to verify it by checking the default branch.

• (cherry picked from commit 26ef84fcce66fcaef81a8b44c2cdc99bb29eb140)

Parent PR: #547