Restrict CI-specific python requirements to Linux

The dependencies declared in ci.requirements.txt are only used in
scripts that we run on the Linux CI.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

scripts/ci.requirements.txt

@@ -2,10 +2,12 @@
 
 -r driver.requirements.txt
 
+# The dependencies below are only used in scripts that we run on the Linux CI.
+
 # Use a known version of Pylint, because new versions tend to add warnings
 # that could start rejecting our code.
 # 2.4.4 is the version in Ubuntu 20.04. It supports Python >=3.5.
-pylint == 2.4.4
+pylint == 2.4.4; platform_system == 'Linux'
 
 # Use a version of mypy that is compatible with our code base.
 # mypy <0.940 is known not to work: see commit
@@ -13,15 +15,14 @@ pylint == 2.4.4
 # mypy >=0.960 is known not to work:
 #   https://github.com/Mbed-TLS/mbedtls-framework/issues/50
 # mypy 0.942 is the version in Ubuntu 22.04.
-mypy == 0.942
+mypy == 0.942; platform_system == 'Linux'
 
 # 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.
 # >=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'
+cryptography >= 35.0.0; platform_system == 'Linux'
 
 # For building `framework/data_files/server9-bad-saltlen.crt` and check python
 # files.
-asn1crypto
+asn1crypto; platform_system == 'Linux'