Set default flavor of buck rust library build operations

Previously e.g. `buck build :cxx` was surprisingly a no-op, requiring
one to build `:cxx#check` or `:cxx#rlib-pic` in order to perform any
compiler invocation at all.

defaults.rust_library.type is a recent feature (landed in facebook/buck@c425eb93e1),
hence the bump of the Buck commit pulled in for CI.

.buckconfig

@@ -18,3 +18,6 @@
       -Clink-arg=-fuse-ld=lld \
       -Crelocation-model=dynamic-no-pic \
       --cap-lints=allow
+
+[defaults.rust_library]
+  type = check

.github/workflows/ci.yml

@@ -58,7 +58,7 @@ jobs:
       - name: Install Buck
         run: |
           mkdir bin
-          wget -q -O bin/buck https://jitpack.io/com/github/facebook/buck/891556d066/buck-891556d066-java11.pex # dev branch from 2020.09.21
+          wget -q -O bin/buck https://jitpack.io/com/github/facebook/buck/a5f0342ae3/buck-a5f0342ae3-java11.pex # dev branch from 2020.10.11
           chmod +x bin/buck
           echo bin >> $GITHUB_PATH
       - name: Install lld