CI: check that the repository doesn't contain any symlink

The Arduino Library Specification forbids symlinks, that's probably the reason why version 6.18.1 is missing from the Library Manager.

.github/workflows/ci.yml

@@ -3,22 +3,24 @@ name: Continuous Integration
 on: [push, pull_request]
 
 jobs:
-  clang-format:
-    name: Clang-Format
+  lint:
+    name: Lint
     runs-on: ubuntu-20.04
     steps:
       - name: Install
         run: sudo apt-get install -y clang-format
       - name: Checkout
         uses: actions/checkout@v2
-      - name: Format
-        run: find src/ extras/ -name '*.[ch]pp' | xargs clang-format -i --verbose --style=file
-      - name: Diff
-        run: git diff --exit-code
+      - name: Symlinks
+        run: find * -type l -printf "::error::%p is a symlink. This is forbidden by the Arduino Library Specification." -exec false {} +
+      - name: Clang-format
+        run: |
+          find src/ extras/ -name '*.[ch]pp' | xargs clang-format -i --verbose --style=file
+          git diff --exit-code
 
   gcc:
     name: GCC
-    needs: clang-format
+    needs: lint
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -66,7 +68,7 @@ jobs:
 
   clang:
     name: Clang
-    needs: clang-format
+    needs: lint
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false