multi-arch-test-build: add fallback if openwrt-version is not found

This might be a rare case, but it happened when I forked the packages
repository under my account, created a PR against the packages repository,
and then someone created a PR against my feature branch, which I had
submitted to the packages repository.

CI/CD was trying to look for target-featurebranch e.g.
aarch64-generic-MyFeatureBranch and thus it failed:

buildx failed with: ERROR: failed to build: failed to solve: ghcr.io/openwrt/sdk:aarch64_generic-MyFeatureBranch: failed to resolve source metadata for ghcr.io/openwrt/sdk:aarch64_generic-MyFeatureBranch: ghcr.io/openwrt/sdk:aarch64_generic-MyFeatureBranch: not found

If there isn't an openwrt-version match, fall back to building for the
OpenWrt master branch (snapshot builds).

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

Author: BKPepe

.github/workflows/multi-arch-test-build.yml

@@ -71,6 +71,13 @@ jobs:
       - name: Determine branch name
         run: |
           BRANCH="${GITHUB_BASE_REF#refs/heads/}"
+          case "$BRANCH" in
+            main|master|openwrt-[0-9]*\.[0-9]*)
+              ;;
+            *)
+              BRANCH="master"
+              ;;
+          esac
           echo "Building for $BRANCH"
           echo "BRANCH=$BRANCH" >> $GITHUB_ENV