Don't install cryptography on the FreeBSD CI

Recent versions of cryptography require a Rust toolchain to install on
FreeBSD, which we do not have set up yet.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/scripts/ci.requirements.txt b/scripts/ci.requirements.txt
index 123b543..4bb41e5 100644
--- a/scripts/ci.requirements.txt
+++ b/scripts/ci.requirements.txt
@@ -17,7 +17,10 @@
 
 # At the time of writing, only needed for tests/scripts/audit-validity-dates.py.
 # It needs >=35.0.0 for correct operation, and that requires Python >=3.6.
-cryptography >= 35.0.0
+# >=35.0.0 also requires Rust to build from source, which we are forced to do on
+# FreeBSD, since PyPI doesn't carry binary wheels for the BSDs.
+# Disable on FreeBSD until we get a Rust toolchain up and running on the CI.
+cryptography >= 35.0.0; platform_system != 'FreeBSD'
 
 # For building `framework/data_files/server9-bad-saltlen.crt` and check python
 # files.