Fix krb5 external test

Fix krb5 external test

Since commit c3845ceba84aab9ddeb43f043549238fd10de63b ("Build file
templates: don't set OPENSSL_{ENGINES,MODULES}") the krb5 external test
has been failing.  This is because it relied on OPENSSL_MODULES already
being set -- even though it did assign to OPENSSL_MODULES itself (and
thus got skipped by the cleanup pass in that commit), it was doing so
only to canonicalize the existing value to an absolute path, not as a de
novo assignment.

Catch up to the rest of the tree and just set it directly as the
"providers" path from the build top (but still canonicalized to an
absolute path).

[extended tests]

Fixes: 11492
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11499)