tests: llext-edk: get the Zephyr SDK path from build_info.yml

The LLEXT EDK test works by packaging an EDK and then testing its
functionality to build the extension in an external CMake build step.
That CMakelists.txt file currently references the required tools using
the ZEPHYR_SDK_INSTALL_DIR environment variable, which must be manually
set by the user.

This change modifies the test to read the newly added 'build_info.yml'
file, generated by Zephyr during the first build step. This allows to
set the ZEPHYR_SDK_INSTALL_DIR environment variable automatically based
on the (possibly auto-discovered) SDK path.

A few minor compliance cleanups are also included.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>