added support of PYPI_VERSION in stack build (#762)

# What does this PR do?

To build a conda env for specific Llama Stack version, e.g.

`PYPI_VERSION=0.0.58 llama stack build --template together --image-type
conda`
will install these in the llamastack-together env:
```
llama_models                             0.0.58
llama_stack                              0.0.58
llama_stack_client                       0.0.58
```
Without `PYPI_VERSION=`, `llama stack build --template together
--image-type conda` installs the latest all.


In short, provide a summary of what this PR does and why. Usually, the
relevant context should be present in a linked issue.

- [ ] Addresses issue (#issue)


## Test Plan

Please describe:
 - tests you ran to verify your changes with result summaries.
 - provide instructions so it can be reproduced.


## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/distribution/build_container.sh

@@ -9,6 +9,7 @@
 LLAMA_MODELS_DIR=${LLAMA_MODELS_DIR:-}
 LLAMA_STACK_DIR=${LLAMA_STACK_DIR:-}
 TEST_PYPI_VERSION=${TEST_PYPI_VERSION:-}
+PYPI_VERSION=${PYPI_VERSION:-}
 BUILD_PLATFORM=${BUILD_PLATFORM:-}
 
 if [ "$#" -lt 4 ]; then
@@ -113,7 +114,12 @@ RUN pip install --no-cache --extra-index-url https://test.pypi.org/simple/ \
   llama-models==$TEST_PYPI_VERSION llama-stack-client==$TEST_PYPI_VERSION llama-stack==$TEST_PYPI_VERSION
 EOF
   else
-    add_to_docker "RUN pip install --no-cache llama-stack"
+    if [ -n "$PYPI_VERSION" ]; then
+      SPEC_VERSION="llama-stack==${PYPI_VERSION} llama-models==${PYPI_VERSION} llama-stack-client==${PYPI_VERSION}"
+    else
+      SPEC_VERSION="llama-stack"
+    fi
+    add_to_docker "RUN pip install --no-cache $SPEC_VERSION"
   fi
 fi