ci: Fix cleanup of ExternalProject custom builds

In CI, we reuse the build directory across different variants (e.g.
RTOSes, target platforms) of a given example, to save time by avoiding
refetching dependencies. However, status information of a previous
ExternalProject custom build is still cached inside build. This causes
ExternalProject to treat the custom build as already done instead of
doing a clean build.

Fix this issue by deleting `__build/_deps/*-build`.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>