Make extension_runner_util install relative

[AdrianLundell]

This removes a reference to the binary tree in the install tree.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❌ 3 New Failures

As of commit 8a0c98d with merge base 5ae40f1 ():

NEW FAILURES - The following jobs have failed:

• Lint / android-java-format / linux-job (gh)
  RuntimeError: Command docker exec -t ec4beae60960512932ade05345249ea328e82054871d5f40529b73e372e94225 /exec failed with exit code 1
• Test CUDA Builds / check-all-cuda-builds (gh)
  Process completed with exit code 1.
• Test CUDA Builds / test-executorch-cuda-build-12.9 / linux-job (gh)
  RuntimeError: Command docker exec -t c8e4f870fcadcee23a59403837bf4aef88efba9cbf35db0b8da29c7537a41920 /exec failed with exit code 1

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[AdrianLundell]

Fails does not look related

[digantdesai]

Assuming CMAKE_BINARY_DIR == CMAKE_INSTALL_PREFIX?

[AdrianLundell]

My intention is to install the lib in CMAKE_INSTALL_PREFIX instead of the binary dir, if someone has a reason for placing it there they can make sure to set CMAKE_BINARY_DIR == CMAKE_INSTALL_PREFIX but I don't see why it would be a requirement.