refactor: updating pipeline variable references

[hallvictoria]

Description

• refactors pipeline variables into a single variable group
  • to add new variables, just need to update the variable group, not each individual pipeline
• testing dev versions of library / extensions is now based on the branch name, instead of a pipeline variable
  • to test python sdk: branch must start with sdk/*
  • to test extensions: branch must start with extensions/*
• removed unnecessary installations to lc tests

Fixes #

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