component/slapos: set SSL_CERT_DIR to use certificates from builddir

In d6f05297 (Do not build openssl 3.5 twice, 2026-04-21), we changed the name installing openssl from openssl to openssl-3.5, which caused the part to be installed in /opt/slapos/parts/openssl-3.5 ( because we use /opt/slapos as rootdir ).

This revealed a problem with obs build, buildout is installing python referencing openssl in rootdir, which before this change used certificates from /opt/slapos/parts/openssl/etc/ssl/certs/ on the host, which worked by chance because this path exists and contains usable certificates on the machine running tests.

After the openssl part renaming, python looks for certificates in /opt/slapos/parts/openssl-3.5/etc/ssl/certs/ which does not exist on the host and this caused certificates verification failures just after rebootstrap when buildout installs eggs for this new python.

The fix here is to set SSL_CERT_DIR for the time of the build, which is similar to how LD_LIBRARY_PATH is set in this profile for some sections, which is necessary to be able to use the parts at build time when they are still in destdir and not yet in rootdir.