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_conda_env.sh

@@ -105,7 +105,13 @@ ensure_conda_env_python310() {
       printf "Installing from LLAMA_STACK_DIR: $LLAMA_STACK_DIR\n"
       $CONDA_PREFIX/bin/pip install --no-cache-dir -e "$LLAMA_STACK_DIR"
     else
-      $CONDA_PREFIX/bin/pip install --no-cache-dir llama-stack
+      PYPI_VERSION="${PYPI_VERSION:-}"
+      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
+      $CONDA_PREFIX/bin/pip install --no-cache-dir $SPEC_VERSION
     fi
 
     if [ -n "$LLAMA_MODELS_DIR" ]; then