Update instructions for build types

[kirklandsign]

Summary: User can use export EXECUTORCH_CMAKE_BUILD_TYPE=Debug

Reviewed By: helunwencser

Differential Revision: D64795144

[pytorch-bot]

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

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This PR needs a release notes: label

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[facebook-github-bot]

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