remove uneeded temporary build artifacts in CI

[sbiscigl]

Description of changes:

During CI we build and install the SDK, however, the integration tests have no install target, so do not clean up the entire build directory. We want the integration test binaries, however we keep the entire build directory in the workspaces. This remove all the unneeded transient files, that sometimes total up to 44 GiB when building in non-unity.

$ du -sh */
8.0K	AWSSDK/
1.7M	CMakeFiles/
73M	crt/
42G	generated/
16M	lib/
155M	src/
648M	tests/

all we really want is the tests directory and the test binaries in them.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

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