Github: Add Shellcheck Action

Summary:
* Add a dedicated shellcheck action to Github actions
* We now get this functionality with the current faster concurrent setup instead of earlier serial pre-commit setup
* All of our existing shell scripts pass shellcheck

Differential Revision: D51608963

.github/workflows/shellcheck.yaml

@@ -0,0 +1,21 @@
+---
+name: Shellcheck
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+jobs:
+  shellcheck:
+    name: Run Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install Shellcheck
+        run: |
+          sudo apt update
+          sudo apt install -y shellcheck
+      - name: Checkout Git repository
+        uses: actions/checkout@v4
+      - name: Run Shellcheck
+        run: |
+          shellcheck **/*.sh

test.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+$result="foo"
+[[ $1 == $result ]] && echo "victory"