Add a gitleaks actions

Because these repositories are in the UW-GAC organization, I also
needed to obtain for a gitleaks license and add it as a secret in
the UW-GAC organization.

.github/workflows/gitleaks.yml

@@ -0,0 +1,19 @@
+name: gitleaks
+on:
+  pull_request:
+  push:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 4 * * *" # run once a day at 4 AM
+jobs:
+  scan:
+    name: gitleaks
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - uses: gitleaks/gitleaks-action@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITLEAKS_LICENSE: ${{ secrets.GITLEAKS_LICENSE}} # Only required for Organizations, not personal accounts.