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>

.github/workflows/scripts/show_build_failures.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+original_exit_code="${ret:-1}"
+log_dir_path="${1:-logs}"
+context="${2:-10}"
+
+show_make_build_errors() {
+	grep -slr 'make\[[[:digit:]]\].*Error [[:digit:]]$' "$log_dir_path" | while IFS= read -r log_file; do
+		printf "====== Make errors from %s ======\n" "$log_file";
+		grep -r -C"$context" 'make\[[[:digit:]]\].*Error [[:digit:]]$' "$log_file" ;
+	done
+}
+
+show_make_build_errors
+exit "$original_exit_code"