Fix commands in the GitHub prerelease (rerun-io#6750)

### What

The command listed on our
[prerelease](https://github.com/rerun-io/rerun/releases/tag/prerelease)
page can, in some circumstances, yield to the most confusing behaviour
([relevant slack
thread](https://rerunio.slack.com/archives/C033K5VS2KD/p1720018840354529)).
This PR fixes the command.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6750?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6750?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/6750)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

.github/workflows/nightly.yml

@@ -402,11 +402,11 @@ jobs:
 
             ## Wheels can be installed with:
             ```
-            pip install --pre -f https://build.rerun.io/commit/${{ env.SHORT_SHA }}/wheels --upgrade rerun-sdk
+            pip install --pre --no-index -f https://build.rerun.io/commit/${{ env.SHORT_SHA }}/wheels --upgrade rerun-sdk
             ```
             or
             ```
-            pip install --pre -f https://github.com/rerun-io/rerun/releases/download/prerelease --upgrade rerun-sdk
+            pip install --pre --no-index -f https://github.com/rerun-io/rerun/releases/download/prerelease --upgrade rerun-sdk
             ```
 
             ## CMake fetch-content for C++ SDK