Release: fix build inside Fedora

```
 => => naming to docker.io/library/fedora                                                                                                                                                                 0.0s
Inside docker: starting build
Cloning into '/build'...
done.
Note: switching to 'ca533a084d7a7636b099de7f6326f549c5251dfc'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

Downloading cpython-3.12.11-linux-x86_64-gnu (download) (29.9MiB)
 Downloading cpython-3.12.11-linux-x86_64-gnu (download)
Using CPython 3.12.11
Resolved 120 packages in 2ms
error: No virtual environment found for Python 3.12; run `uv venv` to create an environment, or pass `--system` to install into a non-virtual environment
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/build-release.sh

@@ -11,6 +11,7 @@ if [ "$1" = "--inside-docker" ]; then
     git config --global --add safe.directory /src/.git
     git clone /src /build
     cd /build || exit
+    uv venv
     uv export --format requirements.txt > /tmp/requirements.txt
     uv pip install -r /tmp/requirements.txt
     ./configure