Fix cysecuretools always being installed

[multiplemonomials]

Summary of changes

The intention of this CMake script was to install cysecuretools only if needed. However, I missed the fact that the block of code I added always gets executed because the script always gets included regardless of target. This PR moves the python package to be installed inside the function that creates the post build function, ensuring that it will only be installed for the specific Cypress targets that need it.

This is an important fix because cysecuretools currently fails to install on Python 3.13 on Macs and Arm Linux.

Impact of changes

Migration actions required

Documentation

none

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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