ci: show build failures directly in job log output

Instead of waiting for complete workflow finish, then downloading the
artifacts, unpacking them and inspecting them, lets try to make the
build failure immediately visible in the log output:

  ====== Make errors from logs/target/linux/compile.txt ======
  * Legacy (non-UHI/non-FIT) Boards
  *
  Support MIPS SEAD-3 boards (LEGACY_BOARD_SEAD3) [N/y/?] (NEW)
  Error in reading or end of file.

  make[6]: *** [scripts/kconfig/Makefile:77: syncconfig] Error 1
  make[5]: *** [Makefile:616: syncconfig] Error 2
  make[4]: *** [Makefile:736: include/config/auto.conf.cmd] Error 2
  make[3]: *** [Makefile:24: build_dir/target-mipsel-openwrt-linux-musl_musl/linux-ramips_mt7620/linux-5.15.62/.modules] Error 2
  make[2]: *** [Makefile:11: compile] Error 2
  time: target/linux/compile#30.09#11.30#37.92

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit f4ca4187cd)

.github/workflows/kernel.yml

@@ -150,22 +150,22 @@ jobs:
       - name: Build tools
         shell: su buildbot -c "sh -e {0}"
         working-directory: openwrt
-        run: make tools/install -j$(nproc) BUILD_LOG=1
+        run: make tools/install -j$(nproc) BUILD_LOG=1 || ret=$? .github/workflows/scripts/show_build_failures.sh
 
       - name: Build toolchain
         shell: su buildbot -c "sh -e {0}"
         working-directory: openwrt
-        run: make toolchain/install -j$(nproc) BUILD_LOG=1
+        run: make toolchain/install -j$(nproc) BUILD_LOG=1 || ret=$? .github/workflows/scripts/show_build_failures.sh
 
       - name: Build Kernel
         shell: su buildbot -c "sh -e {0}"
         working-directory: openwrt
-        run: make target/compile -j$(nproc) BUILD_LOG=1
+        run: make target/compile -j$(nproc) BUILD_LOG=1 || ret=$? .github/workflows/scripts/show_build_failures.sh
 
       - name: Build Kernel Kmods
         shell: su buildbot -c "sh -e {0}"
         working-directory: openwrt
-        run: make package/linux/compile -j$(nproc) BUILD_LOG=1
+        run: make package/linux/compile -j$(nproc) BUILD_LOG=1 || ret=$? .github/workflows/scripts/show_build_failures.sh
 
       - name: Upload logs
         if: failure()