check_format: use GOPATH and add install commands for formatting libr…

…aries (#2608)

The buildifier docs recommend using the go toolchain to install buildifier. I'm not sure if current default just reflects the original authors GOPATH.

I added explicit installation instructions else ./tools/check_format.sh may fail for a new contributor with errors that seemingly indicate an actual format error.

Risk Level: Low

Testing:
passing locally with ./tools/check_format.sh check

Signed-off-by: Christopher M. Luciano <cmluciano@us.ibm.com>

bazel/README.md

@@ -31,6 +31,7 @@ On Ubuntu, run the following commands:
  apt-get install libtool
  apt-get install cmake
  apt-get install realpath
+ apt-get install clang-format-5.0
 ```
 
 On Fedora (maybe also other red hat distros), run the following:
@@ -53,9 +54,10 @@ Envoy compiles and passes tests with the version of clang installed by XCode 8.3
 Apple LLVM version 8.1.0 (clang-802.0.42).
 
 3. Install Golang on your machine. This is required as part of building [BoringSSL](https://boringssl.googlesource.com/boringssl/+/HEAD/BUILDING.md)
-and also for [Buildifer](https://github.com/bazelbuild/buildtools) which is used for formatting bazel BUILD files.
-4. `bazel fetch //source/...` to fetch and build all external dependencies. This may take some time.
-5. `bazel build //source/exe:envoy-static` from the Envoy source directory.
+and also for [Buildifer](https://github.com/bazelbuild/buildtools) which is used for formatting bazel BUILD files. 
+4. `go get github.com/bazelbuild/buildtools/buildifier` to install buildifier
+5. `bazel fetch //source/...` to fetch and build all external dependencies. This may take some time.
+6. `bazel build //source/exe:envoy-static` from the Envoy source directory.
 
 ## Building Bazel with the CI Docker image
 

tools/check_format.py

@@ -17,7 +17,7 @@
 GOOGLE_PROTOBUF_WHITELIST = ('ci/prebuilt', 'source/common/protobuf')
 
 CLANG_FORMAT_PATH = os.getenv("CLANG_FORMAT", "clang-format-5.0")
-BUILDIFIER_PATH = os.getenv("BUILDIFIER", "/usr/lib/go/bin/buildifier")
+BUILDIFIER_PATH = os.getenv("BUILDIFIER_BIN", "$GOPATH/bin/buildifier")
 ENVOY_BUILD_FIXER_PATH = os.path.join(
     os.path.dirname(os.path.abspath(sys.argv[0])), "envoy_build_fixer.py")
 HEADER_ORDER_PATH = os.path.join(