Stop 'Julia auto update' from upgrading Julia version (#1215)

I was wondering why the automated part of #1204 bumps the Julia version
in the Manifest.toml. It is because we didn't run `juliaup default
1.10.0` yet, so 1.10.2 is the default. This fixes that. There is no need
to run all of `pixi run install` for this action.

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: GitHub <noreply@github.com>

.github/workflows/julia_auto_update.yml

@@ -19,8 +19,8 @@ jobs:
           pixi-version: "latest"
       - name: Update Julia manifest file
         run: |
+          pixi run install-julia
           pixi run update-manifest-julia
-          pixi install
       - uses: peter-evans/create-pull-request@v6
         with:
           token: ${{ secrets.GITHUB_TOKEN }}