GH Actions: do not persist credentials

> By default, using `actions/checkout` causes a credential to be persisted in the checked-out repo's `.git/config`, so that subsequent `git` operations can be authenticated.
>
> Subsequent steps may accidentally publicly persist `.git/config`, e.g. by including it in a publicly accessible artifact via `actions/upload-artifact`.
>
> However, even without this, persisting the credential in the `.git/config` is non-ideal unless actually needed.
>
> **Remediation**
>
> Unless needed for `git` operations, `actions/checkout` should be used with `persist-credentials: false`.
>
> If the persisted credential is needed, it should be made explicit with `persist-credentials: true`.

This has now been addressed in all workflows.

Refs:
* https://unit42.paloaltonetworks.com/github-repo-artifacts-leak-tokens/
* https://docs.zizmor.sh/audits/#artipacked

.github/workflows/docs.yaml

@@ -17,6 +17,7 @@ jobs:
         uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
         with:
           fetch-depth: 1
+          persist-credentials: false
       - name: Build Docs
         uses: ./.github/actions/build-docs
       - name: Publish Docs to gh-pages

.github/workflows/tests.yml

@@ -18,6 +18,8 @@ jobs:
     steps:
       - name: Check out code
         uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+        with:
+          persist-credentials: false
 
       - name: Set up PHP
         uses: shivammathur/setup-php@bf6b4fbd49ca58e4608c9c89fba0b8d90bd2a39f # 2.35.5
@@ -58,6 +60,8 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+        with:
+          persist-credentials: false
 
       - name: Install PHP
         uses: shivammathur/setup-php@bf6b4fbd49ca58e4608c9c89fba0b8d90bd2a39f # 2.35.5
@@ -128,6 +132,8 @@ jobs:
     steps:
       - name: Check out code
         uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 # v4.3.0
+        with:
+          persist-credentials: false
 
         # About the "extensions":
         #