ca: Fold a local into its only usage place.

caucase/ca.py

@@ -291,9 +291,6 @@ class CertificateAuthority(object):
     ca_crt = ca_key_pair['crt']
 
     public_key = csr.public_key()
-    subject_key_identifier = x509.SubjectKeyIdentifier.from_public_key(
-      public_key,
-    )
     now = datetime.datetime.utcnow()
     builder = x509.CertificateBuilder(
       subject_name=template_csr.subject,
@@ -311,7 +308,9 @@ class CertificateAuthority(object):
           critical=True, # "MAY appear as critical or non-critical"
         ),
         Extension(
-          subject_key_identifier,
+          x509.SubjectKeyIdentifier.from_public_key(
+            public_key,
+          ),
           critical=False, # "MUST mark this extension as non-critical"
         ),
         Extension(