ci: Pin download-artifact actions sha

Also bumps download-artifact to v7.0.0.

(cherry picked from commit c54af23b41)

.github/workflows/ci.yml

@@ -164,7 +164,7 @@ jobs:
     steps:
     - uses: actions/checkout@v6
     - name: Download installer tarball
-      uses: actions/download-artifact@v6
+      uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
       with:
         name: installer-${{matrix.os}}
         path: out
@@ -288,7 +288,7 @@ jobs:
           repository: NixOS/flake-regressions-data
           path: flake-regressions/tests
       - name: Download installer tarball
-        uses: actions/download-artifact@v7
+        uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
         with:
           name: installer-linux
           path: out