Update instructions for build types

[kirklandsign]

Summary: User can use export EXECUTORCH_CMAKE_BUILD_TYPE=Debug

Reviewed By: helunwencser

Differential Revision: D64795144

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6995

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• [DomainsOnly] Jobs fail with GLIBC version not found

:x: 1 New Failure

As of commit 719114a58562534c5917084fb360e9e4348780fa with merge base 711f1c23d32acd798dabf0028b31e8fcb2c2487f ():

NEW FAILURE - The following job has failed:

* [Check Labels / Check labels](https://hud.pytorch.org/pr/pytorch/executorch/6995#33292986844) ([gh](https://github.com/pytorch/executorch/actions/runs/11943614567/job/33292986844)) `RuntimeError: Error checking labels: PR does not have required labels`

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D64795144

[github-actions[bot]]

This PR needs a release notes: label

If your changes are user facing and intended to be a part of release notes, please use a label starting with release notes:.

If not, please add the topic: not user facing label.

To add a label, you can comment to pytorchbot, for example @pytorchbot label "topic: not user facing"

For more information, see https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D64795144