Use uv pip install instead of pip install (#921)

## What does this PR do? 

See issue: #747 -- `uv` is just plain better. This PR does the bare
minimum of replacing `pip install` by `uv pip install` and ensuring `uv`
exists in the environment.

## Test Plan 

First: create new conda, `uv pip install -e .` on `llama-stack` -- all
is good.
Next: run `llama stack build --template together` followed by `llama
stack run together` -- all good
Next: run `llama stack build --template together --image-name yoyo`
followed by `llama stack run together --image-name yoyo` -- all good
Next: fresh conda and `uv pip install -e .` and `llama stack build
--template together --image-type venv` -- all good.

Docker: `llama stack build --template together --image-type container`
works!

.github/workflows/pre-commit.yml

@@ -11,10 +11,10 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
+        uses: actions/checkout@v4
 
       - name: Set up Python
-        uses: actions/setup-python@f677139bbe7f9c59b41e40162b753c062f5d49a3 # v5.2.0
+        uses: actions/setup-python@v5
         with:
           python-version: '3.11'
           cache: pip
@@ -22,4 +22,4 @@ jobs:
             **/requirements*.txt
             .pre-commit-config.yaml
 
-      - uses: pre-commit/action@2c7b3805fd2a0fd8c1884dcaebf91fc102a13ecd #v3.0.1
+      - uses: pre-commit/action@v3.0.1