Fix CI: bump VSIX build to Python 3.10 to support importlib-metadata 9.0.0 (#25879)

`importlib-metadata==9.0.0` requires Python >=3.10, but the VSIX build
job was running under Python 3.9 — causing pip to reject the package
entirely. Since Python 3.9 is EOL, the fix is to upgrade the build
environment rather than downgrade the package.

## Changes

- **`.github/actions/build-vsix/action.yml`**: Updated the JediLSP build
step from Python 3.9 to Python 3.10, unblocking
`importlib-metadata==9.0.0`.

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<details>

<summary>Original prompt</summary>

> Fix the failing GitHub Actions workflow "PR/CI Check / Create VSIX
(ubuntu-latest, x86_64-unknown-linux-musl, alpine-x64) (pull_request)".
> Analyze the workflow logs, identify the root cause of the failure, and
implement a fix.
> Job ID: 68304650771
> Job URL:
/microsoft/vscode-python/actions/runs/23474615229/job/68304650771?pr=25878


</details>



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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rzhao271 <7199958+rzhao271@users.noreply.github.com>

Author: Copilot

.github/actions/build-vsix/action.yml

@@ -31,10 +31,10 @@ runs:
       uses: dtolnay/rust-toolchain@stable
 
     # Jedi LS depends on dataclasses which is not in the stdlib in Python 3.7.
-    - name: Use Python 3.9 for JediLSP
+    - name: Use Python 3.10 for JediLSP
       uses: actions/setup-python@v6
       with:
-        python-version: 3.9
+        python-version: "3.10"
         cache: 'pip'
         cache-dependency-path: |
           requirements.txt