openwrt: Handle if ${remote} is a tag ref instead of a branch

E.g., for the makefile target output-source_v21.02.0-rc1 -- you cannot
check it out as source/v21.02.0-rc1, but you can check it out as just
v21.02.0-rc1.

openwrt-builder-commit.dockerfile

@@ -11,7 +11,9 @@ RUN cd ${openwrt_dir} \
 
 RUN cd ${openwrt_dir} \
     && git fetch --all \
-    && git checkout ${remote}/${commit} \
+    && git checkout ${remote}/${commit} || git checkout "${commit}"
+
+RUN cd ${openwrt_dir} \
     && ./scripts/feeds update -a \
     && ./scripts/feeds install -a