github-ci: add a job with luacheck

luacheck config has been already added in commit
'lua: fix code style in test scripts' (17b725f).
As well as fixes for warnings found by luacheck.

Part of #142

.github/workflows/check.yaml

@@ -0,0 +1,26 @@
+name: Run checks
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  luacheck:
+    runs-on: ubuntu-latest
+    if: |
+      github.event_name == 'push' ||
+      github.event_name == 'pull_request' &&
+      github.event.pull_request.head.repo.full_name != github.repository
+    steps:
+    - uses: actions/checkout@master
+
+    - name: Setup Tarantool
+      uses: tarantool/setup-tarantool@v1
+      with:
+        tarantool-version: '2.8'
+
+    - name: Setup luacheck
+      run: tarantoolctl rocks install luacheck 0.25.0
+
+    - name: Run luacheck
+      run: ./.rocks/bin/luacheck .