Fix EFI variable, update CI to use 20.04

.gitlab-ci.yml

-image: "ubuntu:18.04"
+image: "ubuntu:20.04"
 
 variables:
   GIT_STRATEGY: "pull"
@@ -7,6 +7,7 @@ variables:
 
 before_script:
  - |
+    export DEBIAN_FRONTEND=noninteractive
     apt-get update -qq &&
     apt-get install -qq \
         bison \

mk/config.mk

@@ -53,7 +53,7 @@ INSTALLER=\
  	installer/target/release/redox_installer $(INSTALLER_FLAGS)
 
 ## Bootloader variables
-EFI_TARGET=$(ARCH)-efi-pe
+EFI_TARGET=$(ARCH)-unknown-uefi
 
 ## Cross compiler variables
 AR=$(TARGET)-gcc-ar