[bootloader] Convert to str

- The output of subprocess is a bytes object, which needs to
   be decoded so we can use it like a regular string (alternatively,
   we could have changed more code to manipulate bytes, but eventually
   we need a string to pass to a subsequent command anyway).

src/modules/bootloader/main.py

@@ -366,10 +366,10 @@ def install_secureboot(efi_directory):
     # of that tuple.
     efi_drive = subprocess.check_output([
         libcalamares.job.configuration["grubProbe"],
-        "-t", "drive", "--device-map=", install_efi_directory])
+        "-t", "drive", "--device-map=", install_efi_directory]).decode("ascii")
     efi_disk = subprocess.check_output([
         libcalamares.job.configuration["grubProbe"],
-        "-t", "disk", "--device-map=", install_efi_directory])
+        "-t", "disk", "--device-map=", install_efi_directory]).decode("ascii")
 
     efi_drive_partition = efi_drive.replace("(","").replace(")","").split(",")[1]
     # Get the first run of digits from the partition