Ensure that GOBIN is not set when installing a golang hook

If GOBIN is set, it will be used as the install path instead of the first item
from GOPATH followed by "/bin".  If it is used, commands will not be isolated
between different repos.

pre_commit/languages/golang.py

@@ -7,6 +7,7 @@ import sys
 import pre_commit.constants as C
 from pre_commit import git
 from pre_commit.envcontext import envcontext
+from pre_commit.envcontext import UNSET
 from pre_commit.envcontext import Var
 from pre_commit.languages import helpers
 from pre_commit.util import clean_path_on_failure
@@ -21,6 +22,7 @@ healthy = helpers.basic_healthy
 
 def get_env_patch(venv):
     return (
+        ('GOBIN', UNSET),
         ('PATH', (os.path.join(venv, 'bin'), os.pathsep, Var('PATH'))),
     )
 
@@ -69,6 +71,7 @@ def install_environment(prefix, version, additional_dependencies):
         else:
             gopath = directory
         env = dict(os.environ, GOPATH=gopath)
+        env.pop('GOBIN', None)
         cmd_output('go', 'get', './...', cwd=repo_src_dir, env=env)
         for dependency in additional_dependencies:
             cmd_output('go', 'get', dependency, cwd=repo_src_dir, env=env)