package/python-pyicu: fix build

package/python-pyicu: fix build

Fixes:
http://autobuild.buildroot.net/results/977/977c043615603dd3541e23b81c5f11374606aff6/

Set environment variables to cross-compile friendly values, this prevents
icu-config picking up host libraries:
https://gitlab.pyicu.org/main/pyicu/-/blob/main/setup.py#L89
https://gitlab.pyicu.org/main/pyicu/-/blob/main/setup.py#L179
https://gitlab.pyicu.org/main/pyicu/-/blob/main/setup.py#L216

This follows the upstream build docs:
https://gitlab.pyicu.org/main/pyicu#building-pyicu

Signed-off-by: Bernd Kuhls <[email protected]>
Signed-off-by: Arnout Vandecappelle <[email protected]>