fix: usage of `a deprecated Node.js version` in CI (#8506)

## Summary

[`ci.yml`](https://github.com/astral-sh/uv/blob/main/.github/workflows/ci.yml)
uses
[`gabrielfalcao/pyenv-action@v18`](https://github.com/gabrielfalcao/pyenv-action/),
which [uses `a deprecated Node.js
version`](https://github.com/astral-sh/uv/actions/runs/11483963555).

This pull request aims to remove any usage of `a deprecated Node.js
version` from
[`ci.yml`](https://github.com/astral-sh/uv/blob/main/.github/workflows/ci.yml).

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

I attempted to test this but [canceled the run on my
fork](https://github.com/hamirmahal/uv/actions/runs/11484989508/job/31964058415)
after it waited for a runner for over 10 minutes with no result.

<!-- How was it tested? -->

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

.github/workflows/ci.yml

@@ -1606,9 +1606,23 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: "Install pyenv"
-        uses: "gabrielfalcao/pyenv-action@v18"
-        with:
-          default: 3.9.7
+        run: |
+          # Install pyenv
+          curl https://pyenv.run | bash
+
+          # Set up environment variables for current step
+          export PYENV_ROOT="$HOME/.pyenv"
+          export PATH="$PYENV_ROOT/bin:$PATH"
+          eval "$(pyenv init -)"
+
+          # Install Python 3.9
+          pyenv install 3.9
+          pyenv global 3.9
+
+          # Make environment variables persist across steps
+          echo "PYENV_ROOT=$HOME/.pyenv" >> $GITHUB_ENV
+          echo "$HOME/.pyenv/bin" >> $GITHUB_PATH
+          echo "$HOME/.pyenv/shims" >> $GITHUB_PATH
 
       - name: "Download binary"
         uses: actions/download-artifact@v4