GHA: set persist-credentials: false

Suggested by zizmor GHA analysis tool.

Also:
- Move GH variables within single-quotes.
- Prefer single-quotes in shell code. (tidy-up)

Ref: https://github.com/actions/checkout/issues/485
Ref: https://github.com/actions/checkout/pull/1687
Ref: https://woodruffw.github.io/zizmor/

Closes #15746

.github/workflows/macos.yml

@@ -208,6 +208,8 @@ jobs:
           fi
 
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        with:
+          persist-credentials: false
 
       - name: 'toolchain versions'
         run: |
@@ -416,6 +418,8 @@ jobs:
           while [[ $? == 0 ]]; do for i in 1 2 3; do brew update && brew bundle install --no-lock --file /tmp/Brewfile && break 2 || { echo Error: wait to try again; sleep 10; } done; false Too many retries; done
 
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        with:
+          persist-credentials: false
 
       - name: 'toolchain versions'
         run: |