CI: Split off lint as a separate job and add shellcheck

.github/workflows/lint.yml

@@ -0,0 +1,18 @@
+name: Lint code
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.10'
+      - name: Install flake8
+        run: pip install flake8
+      - name: Install shellcheck
+        run: sudo apt-get install shellcheck
+      - name: Lint code
+        run: ./test/lint/lint-all.sh

.github/workflows/unittests.yml

@@ -35,8 +35,6 @@ jobs:
         run: |
           ./install.sh --develop --with-qt
           ./jmvenv/bin/python -m pip install --upgrade pip
-      - name: Lint with flake8
-        run: source ./jmvenv/bin/activate && ./test/lint/lint-python.sh
       - name: Cache bitcoind
         uses: actions/cache@v3
         env: