Use uv pip install ipykernel for dev environments

[r4victor]

Replaces pip install with uv pip install for installing ipykernel in dev environments to speed up dev environment startup time. The commands have the same effect as uv pip installs to the same Python's environment (/dstack/venv in dstack images). Falls back to pip install if uv is missing.

Also makes CLI run waiting interval small (1s vs 5s) for a few first attempts to reduce the CLI update lag in case of fast run startup.

# time pip install --no-cache-dir ipykernel
real	0m5.947s
user	0m4.957s
sys	0m0.671s

# time uv pip install --no-cache-dir ipykernel
real	0m1.377s
user	0m0.734s
sys	0m0.828s