sdk: Forward SIGNER environment variable

Some of the signing may happen inside the SDK container, so make sure
to forward the SIGNER environment variable, as it will be used by the
signing function, when it's introduced.

sdk_lib/90_env_keep

@@ -5,4 +5,5 @@ Defaults env_keep += "FLATCAR_BUILD_ID COREOS_OFFICIAL \
                       GNUPGHOME GPG_AGENT_INFO SSH_AUTH_SOCK \
                       BOTO_PATH GOOGLE_APPLICATION_CREDENTIALS \
                       USE FEATURES PORTAGE_USERNAME FORCE_STAGES \
+                      SIGNER \
                       all_proxy ftp_proxy http_proxy https_proxy no_proxy"

sdk_lib/sdk_container_common.sh

@@ -201,6 +201,7 @@ function setup_sdk_env() {
         GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME \
         GIT_PROXY_COMMAND GIT_SSH RSYNC_PROXY \
         GPG_AGENT_INFO FORCE_STAGES \
+        SIGNER \
         all_proxy ftp_proxy http_proxy https_proxy no_proxy; do
 
         if [ -n "${!var:-}" ] ; then