Fix the schema diff CI breaking when the Rust module was changed. (#20117)

Follows: #20027

Noticed in https://github.com/element-hq/synapse/pull/20003

The problem was that `--no-root` prevents reinstalling the root project
(Synapse).
However, since we just did a `git checkout`, we need to reinstall the
root project in case the Rust code changed, as the
`poetry install` command is what causes the Rust to be recompiled.

Doing otherwise causes the 'Rust module outdated' error when importing
`synapse`.

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

.ci/scripts/schema_diff.py

@@ -193,7 +193,7 @@ def main() -> None:
             # Refresh dependencies
             print("Installing dependencies for base commit...", file=sys.stderr)
             subprocess.run(
-                ["poetry", "install", "--no-root", "--extras", "postgres"],
+                ["poetry", "install", "--extras", "postgres"],
                 cwd=REPO_ROOT,
                 check=True,
                 # Poetry install is noisy, so pipe its stdout to stderr

changelog.d/20117.misc

@@ -0,0 +1 @@
+Fix the schema diff CI breaking when the Rust module was changed.