fix: stop overriding CURRENT_PROJECT_VERSION with the CI run number

assemble was passing CURRENT_PROJECT_VERSION=$BUILD_NUMBER (github.run_
number-style CI counter) as an xcarg, silently replacing the project's
real, manually-managed version (e.g. '2026.09.11.0.0.1') with an
unrelated small integer - the just-uploaded TestFlight build showed up
as '0.0.1 (51)' instead of the intended version. Removed the override
and the now-unused BUILD_NUMBER env var in beta.yml; CI now always
builds with whatever CURRENT_PROJECT_VERSION is committed in the
project. Re-running CI against the same commit without bumping it first
will now get a clear 'duplicate build number' rejection from Apple
instead of silently uploading under the wrong version.

.gitea/workflows/beta.yml

@@ -9,9 +9,6 @@ jobs:
   build-and-upload:
     runs-on: macos-build
 
-    env:
-      BUILD_NUMBER: ${{ github.run_number }}
-
     steps:
       - name: Checkout repository (workaround - Gitea git-upload-pack bug, see go-gitea/gitea#21819)
         run: |