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Fix editable install by unsetting `build_ext.copy_extensions_to_source` (#1710)

This method was the culprit for the recent editable install breakage,
since it just tries to copy the generated extension file without checking
its existence.

Since the `BazelExtension` uses a non-standard location to store the
build artifacts, calling the copy method fails the build since the
extension is not found in the expected location.

But, since we already copy the file into the source tree as part of the
`BazelExtension.bazel_build` method, it's fine - the extension appears
in the right place, and the egg info is generated correctly as well.

This method also does not affect the general install, so it solves the
editable problem without regressing the fixed install.

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>