Fix build.lock file so charm can build

Due to https://github.com/juju/charm-tools/issues/604 writing the
build.lock fails when the interface:ceph-mds file is in the build.
Update the build.lock with the commit as the branch.

Change-Id: I4ad693c87e2dbcdcec2eba316ababdd5449e5e67

requirements.txt

@@ -9,6 +9,7 @@
 setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb244e9b27bc56750c44b7c85
 
 # Build requirements
+cffi==1.14.6; python_version < '3.6'  # cffi 1.15.0 drops support for py35.
 charm-tools==2.8.3
 
 simplejson

src/build.lock

@@ -53,7 +53,7 @@
       "item": "interface:ceph-mds",
       "url": "https://opendev.org/openstack/charm-interface-ceph-client.git",
       "vcs": null,
-      "branch": null,
+      "branch": "3ca251fb01a7cd51d2a0ee2b9b66647c8ffe891f",
       "commit": "3ca251fb01a7cd51d2a0ee2b9b66647c8ffe891f"
     },
     {
@@ -195,4 +195,4 @@
       "version": "0.7.2"
     }
   ]
-}
+}