build with uv rather than pip

The build command was `pip install .`, which is what the dora docs use because
`--uv` rewrites it. Spelling it `uv pip install .` means the dataflow never
installs through pip at all, whether or not `--uv` was passed. The cost is that
a node then needs a virtualenv to install into, so dataflows must be run with
`--uv`; AGENTS.md says so.

Note that the PR #3 branch still carries `pip install .`; landing this branch
on top of it is what corrects that.

dataflow.yml

@@ -4,7 +4,7 @@
 #
 # It lives at the repository root rather than beside tests/source.py because a
 # node's working directory is the directory of the dataflow file, and both
-# nodes build with `pip install .`, which needs pyproject.toml in that
+# nodes build with `uv pip install .`, which needs pyproject.toml in that
 # directory.
 #
 # Both nodes are built from the checkout with `path:` + `build:`, which is what
@@ -38,7 +38,7 @@ nodes:
     args: tests/source.py
     # The source needs dora-rs, numpy and pyarrow; installing the project gives
     # it exactly the versions pinned in uv.lock.
-    build: pip install .
+    build: uv pip install .
     inputs:
       tick: dora/timer/millis/33
     outputs:
@@ -57,7 +57,7 @@ nodes:
 
   - id: view
     path: dora-image-view
-    build: pip install .
+    build: uv pip install .
     inputs:
       image: source/image
       depth: source/depth