libva: fix runner label — actrunner-debian-aarch64-bohr

The previous commit used `debian-aarch64-bohr` for the runs-on
label, but the actual registered label is the full
`actrunner-debian-aarch64-bohr` (per @marfrit on the PR thread).
Without the `actrunner-` prefix the job would have sat in the queue
forever waiting for a runner that doesn't exist.

Renames the label in three places:

  - .gitea/workflows/build.yml — the runs-on directive itself.
  - debian/.../build-deb.sh — the comment referencing the expected
    runner, and the error message printed when the ABI sanity check
    trips (so a future debug session lands on the right runner name).
  - debian/.../changelog — the -2 entry's prose description.

Pure label rename; no behavioral or build-graph change.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -976,7 +976,7 @@ jobs:
     # __vaDriverInit_1_23, which trixie's libva 2.22 runtime cannot bind: the
     # .deb installed but vaInitialize() returned -1 on every host.  A native
     # trixie runner avoids the cross-distro ABI skew entirely.
-    runs-on: debian-aarch64-bohr
+    runs-on: actrunner-debian-aarch64-bohr
     continue-on-error: true
     steps:
       - uses: actions/checkout@v4