Use and suggest repo_env for compiler choice

Setting CC only for repository rules suffices to influence the compiler
chosen by @local_config_cc, but is more economic with respect to caching
than setting it for all actions. CXX is not used by Bazel.

README.md

@@ -71,8 +71,7 @@ It is best to create command shorthands for the fuzzing configurations you will
 
 ```
 # Force the use of Clang for C++ builds.
-build --action_env=CC=clang
-build --action_env=CXX=clang++
+build --repo_env=CC=clang
 
 # Define the --config=asan-libfuzzer configuration.
 build:asan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine=@rules_fuzzing//fuzzing/engines:libfuzzer
@@ -168,8 +167,7 @@ To use Jazzer, it is convenient to also define a `.bazelrc` configuration, simil
 
 ```
 # Force the use of Clang for all builds (Jazzer requires at least Clang 9).
-build --action_env=CC=clang
-build --action_env=CXX=clang++
+build --repo_env=CC=clang
 
 # Define --config=jazzer for Jazzer without sanitizer (Java only).
 build:jazzer --@rules_fuzzing//fuzzing:java_engine=@rules_fuzzing//fuzzing/engines:jazzer