builder/remotecontext/git: allow building on go1.18

cmd.Environ() is new in go1.19, and not needed for this specific case.
Without this, trying to use this package in code that uses go1.18 will fail;

    builder/remotecontext/git/gitutils.go:216:23: cmd.Environ undefined (type *exec.Cmd has no field or method Environ)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/remotecontext/git/gitutils.go

@@ -213,7 +213,7 @@ 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.Environ(), "GIT_PROTOCOL_FROM_USER=0")
+	cmd.Env = append(os.Environ(), "GIT_PROTOCOL_FROM_USER=0")
 
 	if repo.isolateConfig {
 		cmd.Env = append(cmd.Env,