ci: Remove nix actions cache and prepopulate job

The nix actions cache currently leads to repeated "no space left on
devices" errors for jobs in CI.

The prepopulate job is useless without the nix actions cache, so it
will go away at the same time.

.github/workflows/loadtest.yaml

@@ -26,7 +26,6 @@ jobs:
         uses: ./.github/actions/setup-nix
         with:
           tools: loadtest
-          cache-id: test-loadtest
       - uses: actions-ecosystem/action-get-latest-tag@v1
         id: get-latest-tag
         with:
@@ -58,7 +57,6 @@ jobs:
         uses: ./.github/actions/setup-nix
         with:
           tools: loadtest
-          cache-id: test-loadtest
       - name: Run loadtest
         run: |
           postgrest-loadtest-against ${{ steps.get-latest-tag.outputs.tag }}