Add customizable binaries support

[scaliby]

Description

Add customizable binaries support

Issue

b/487987761

Testing

Tested manually in my terminal.

[jamOne-]

WDYT about this Gemini review:

  1. Handle Path Expansion (High Impact):
     Shell-specific variables (like $HOME) and tildes (~) are frequently used in
  configuration files for paths, but they are not automatically expanded when subprocess
  searches PATH.
      * Fix: Apply os.path.expanduser and os.path.expandvars to the custom_binaries_path
        before prepending it to os.environ['PATH']. If users specify multiple custom paths
        separated by os.pathsep (e.g., :), split, expand each part, and rejoin them.

[scaliby]

WDYT about this Gemini review:

  1. Handle Path Expansion (High Impact):
     Shell-specific variables (like $HOME) and tildes (~) are frequently used in
  configuration files for paths, but they are not automatically expanded when subprocess
  searches PATH.
      * Fix: Apply os.path.expanduser and os.path.expandvars to the custom_binaries_path
        before prepending it to os.environ['PATH']. If users specify multiple custom paths
        separated by os.pathsep (e.g., :), split, expand each part, and rejoin them.

The expansion works when you set the flag. In fact I tried to override bins with this command:

xpk config set custom-binaries-path $PWD/bin_test

Which set the following property in config:

version: v1
configs:
  client-id: XXX
  custom-binaries-path: /home/scaliby/xpk/bin_test

So the expansion happened at the bash execution time, when setting the property. Do we need to do it as well at runtime?
@jamOne- WDYT?

[jamOne-]

Right, lgtm. Thanks for checking