Revert "more workarounds for nested swift-builds"

[ktoso]

This reverts commit 06947f1.

This PR is to see if the issue is resolved in more recent nightlies.

[marcprux]

We had the same issue with Skip's invocation of Android builds.

We addressed this by filtering everything out of the environment except for a pre-approved-list of variables when running from Xcode (as determined by checking the XCODE_VERSION_MAJOR environment variable).

In addition to filtering out SDKROOT, this can also be important for ensuring that incremental builds work, since some of Xcode's ever-changing environment variables (like LLBUILD_TASK_ID) will cause Swift to think that a complete rebuild is necessary every time. I'm not sure if this is still true with swiftbuild, but it was leading to very slow rebuilds for us.

[ktoso]

Hah, interesting, thanks for the info Marc! I wonder then if we just need t keep this...