Include Git branch in CircleCI cache keys and bump key version.

This should resolve an issue affecting the 2.7.5 release, where an old
federation-2/target/ cache was restored from an unrelated PR branch
build (because Cargo.lock was the same), causing the compiled supergraph
binary to call op_composition_result with a string rather than an object
in some cases, triggering a panic on the Rust side.

I would have liked to disable this caching completely when publishing an
official release, to prioritize correctness over build speed, but it
doesn't seem like conditional caching is supported by CircleCI:
https://circleci.com/docs/caching/#clearing-cache

We could potentially run `cargo clean` conditionally for release builds,
after restore_cache (thereby throwing away the cache), but that doesn't
seem ideal.

.circleci/config.yml

@@ -418,14 +418,14 @@ commands:
     steps:
       - restore_cache:
           keys:
-            - rust-target-v2-<< parameters.platform >>-{{ checksum "Cargo.lock" }}
+            - &target-cache-key rust-target-v3-<< parameters.platform >>-{{ .Branch }}-{{ checksum "Cargo.lock" }}
 
       - run:
           command: cargo xtask << parameters.command >> << parameters.options >>
           working_directory: << parameters.working_directory >>
 
       - save_cache:
-          key: rust-target-v2-<< parameters.platform >>-{{ checksum "Cargo.lock" }}
+          key: *target-cache-key
           paths:
             - target/