chore(ci): address linting findings in sync-python-releases.yml (#15117)

## Summary

Continuing to burn these down, one at a time.

This eliminates some implicit credentials, moves a permission block to
its minimum scope of effect, and removes an (unexploitable) template
expansion.

@konstin to answer your earlier question: I tried `permissions:` this
time and got a syntax warning, so I suspect it _needs_ to be an empty
mapping object here 🙂

## Test Plan

I will manually dispatch this workflow once the PR is open.

Edit: Dispatched:
https://github.com/astral-sh/uv/actions/runs/16787049700/job/47540074086

Signed-off-by: William Woodruff <william@astral.sh>

.github/workflows/sync-python-releases.yml

@@ -7,16 +7,19 @@ on:
   schedule:
     - cron: "0 0 * * *"
 
-permissions:
-  contents: write
-  pull-requests: write
+permissions: {}
 
 jobs:
   sync:
     if: github.repository == 'astral-sh/uv'
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
       - uses: astral-sh/setup-uv@e92bafb6253dcd438e0484186d7669ea7a8ca1cc # v6.4.3
         with:
           version: "latest"
@@ -29,7 +32,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Sync Sysconfig Targets
-        run: ${{ github.workspace }}/crates/uv-dev/sync_sysconfig_targets.sh
+        run: ${GITHUB_WORKSPACE}/crates/uv-dev/sync_sysconfig_targets.sh
         working-directory: ./crates/uv-dev
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}