builder: fix running git commands on Windows

Setting cmd.Env overrides the default of passing through the parent
process' environment, which works out fine most of the time, except when
it doesn't. For whatever reason, leaving out all the environment causes
git-for-windows sh.exe subprocesses to enter an infinite loop of
access violations during Cygwin initialization in certain environments
(specifically, our very own dev container image).

Signed-off-by: Cory Snider <csnider@mirantis.com>

builder/remotecontext/git/gitutils.go

@@ -212,7 +212,7 @@ func (repo gitRepo) gitWithinDir(dir string, args ...string) ([]byte, error) {
 	cmd := exec.Command("git", args...)
 	cmd.Dir = dir
 	// Disable unsafe remote protocols.
-	cmd.Env = append(cmd.Env, "GIT_PROTOCOL_FROM_USER=0")
+	cmd.Env = append(cmd.Environ(), "GIT_PROTOCOL_FROM_USER=0")
 
 	if repo.isolateConfig {
 		cmd.Env = append(cmd.Env,